Systems and methods for fabrication of superconducting integrated circuits

ABSTRACT

Various techniques and apparatus permit fabrication of superconductive circuits. A niobium/aluminum oxide/niobium trilayer may be formed and individual Josephson Junctions (JJs) formed. A protective cap may protect a JJ during fabrication. A hybrid dielectric may be formed. A superconductive integrated circuit may be formed using a subtractive patterning and/or additive patterning. A superconducting metal layer may be deposited by electroplating and/or polished by chemical-mechanical planarization. The thickness of an inner layer dielectric may be controlled by a deposition process. A substrate may include a base of silicon and top layer including aluminum oxide. Depositing of superconducting metal layer may be stopped or paused to allow cooling before completion. Multiple layers may be aligned by patterning an alignment marker in a superconducting metal layer.

BACKGROUND

Field

The present systems and methods relate to the fabrication of integratedcircuits for superconducting applications.

Approaches to Quantum Computation

There are several general approaches to the design and operation ofquantum computers. One such approach is the “circuit” or “gate” model ofquantum computation. In this approach, qubits are acted upon bysequences of logical gates that are the compiled representation of analgorithm. Much research has been focused on developing qubits withsufficient coherence to form the basic elements of circuit model quantumcomputers.

Another approach to quantum computation involves using the naturalphysical evolution of a system of coupled quantum devices as acomputational system. This approach may not make use of quantum gatesand circuits. Instead, the computational system may start from a knowninitial Hamiltonian with an easily accessible ground state and becontrollably guided to a final Hamiltonian whose ground state representsthe answer to a problem. This approach does not typically require longqubit coherence times and may be more robust than the circuit model.Examples of this type of approach include adiabatic quantum computationand quantum annealing.

Quantum Processor

Quantum computations may be performed using a quantum processor, such asa superconducting quantum processor. A superconducting quantum processormay comprise a superconducting integrated circuit including a number ofqubits and associated local bias devices, for instance two or moresuperconducting qubits. Further details on systems and methods ofexemplary superconducting quantum processors that may be fabricatedaccording to the present systems and methods are described in U.S. Pat.Nos. 7,135,701, 7,418,283, 7,533,068, 7,619,437, 7,639,035, 7,898,282,8,008,942, 8,190,548, 8,195,596, 8,283,943, and US Patent ApplicationPublication 2011-0022820, each of which is incorporated herein byreference in its entirety.

Superconducting Qubits

Superconducting qubits are a type of superconducting quantum device thatcan be included in a superconducting integrated circuit. Superconductingqubits can be separated into several categories depending on thephysical property used to encode information. For example, they may beseparated into charge, flux and phase devices. Charge devices store andmanipulate information in the charge states of the device. Flux devicesstore and manipulate information in a variable related to the magneticflux through some part of the device. Phase devices store and manipulateinformation in a variable related to the difference in superconductingphase between two regions of the phase device. Recently, hybrid devicesusing two or more of charge, flux and phase degrees of freedom have beendeveloped.

Superconducting integrated circuits may include single flux quantum(SFQ) devices. The integration of SFQ devices with superconductingqubits is discussed in, for example, U.S. Pat. Nos. 7,876,248,8,035,540, 8,098,179, and US Patent Publication Number 2011-0065586,each of which is incorporated herein by reference in its entirety.

Superconducting Processor

A computer processor may take the form of a superconducting processor,where the superconducting processor may not be a quantum processor inthe traditional sense. For instance, some embodiments of asuperconducting processor may not focus on quantum effects such asquantum tunneling, superposition, and entanglement but may ratheroperate by emphasizing different principles, such as for example theprinciples that govern the operation of classical computer processors.However, there may still be certain advantages to the implementation ofsuch superconducting “classical” processors. Due to their naturalphysical properties, superconducting classical processors may be capableof higher switching speeds and shorter computation times thannon-superconducting processors, and therefore it may be more practicalto solve certain problems on superconducting classical processors. Thepresent systems and methods are particularly well-suited for use infabricating both superconducting quantum processors and superconductingclassical processors.

Integrated Circuit Fabrication

Traditionally, the fabrication of superconducting integrated circuitshas not been performed at state-of-the-art semiconductor fabricationfacilities. This may be due to the fact that some of the materials usedin superconducting integrated circuits can contaminate the semiconductorfacilities. For instance, gold may be used as a resistor insuperconducting circuits, but gold can contaminate a fabrication toolused to produce CMOS wafers in a semiconductor facility. Consequently,superconducting integrated circuits containing gold are typically notprocessed by tools which also process CMOS wafers.

Superconductor fabrication has typically been performed in researchenvironments where standard industry practices could be optimized forsuperconducting circuit production. Superconducting integrated circuitsare often fabricated with tools that are traditionally used to fabricatesemiconductor chips or integrated circuits. Due to issues unique tosuperconducting circuits, not all semiconductor processes and techniquesare necessarily transferable to superconductor chip manufacture.Transforming semiconductor processes and techniques for use insuperconductor chip and circuit fabrication often requires changes andfine adjustments. Such changes and adjustments typically are not obviousand may require a great deal of experimentation. The semiconductorindustry faces problems and issues not necessarily related to thesuperconducting industry. Likewise, problems and issues that concern thesuperconducting industry are often of little or no concern in standardsemiconductor fabrication.

Any impurities within superconducting chips may result in noise whichcan compromise or degrade the functionality of the individual devices,such as superconducting qubits, and of the superconducting chip as awhole. Since noise is a serious concern to the operation of quantumcomputers, measures should be taken to reduce dielectric noise whereverpossible.

The art of integrated circuit fabrication typically involves multipleprocesses that may be sequenced and/or combined to produce a desiredeffect. Exemplary systems and methods for superconducting integratedcircuit fabrication that may be combined, in whole or in part, with atleast some embodiments of the present systems and methods are describedin US Patent Publication Number 2011-0089405, which is incorporatedherein by reference in its entirety.

Etching

Etching removes layers of, for example, substrates, dielectric layers,oxide layers, electrically insulating layers and/or metal layersaccording to desired patterns delineated by photoresists or othermasking techniques. Two exemplary etching techniques are wet chemicaletching and dry chemical etching.

Wet chemical etching or “wet etching” is typically accomplished bysubmerging a wafer in a corrosive bath such as an acid bath. In general,etching solutions are housed in polypropylene, temperature-controlledbaths. The baths are usually equipped with either a ring-type plenumexhaust ventilation or a slotted exhaust at the rear of the etchstation. Vertical laminar-flow hoods are typically used to supplyuniformly-filtered, particulate-free air to the top surface of the etchbaths.

Dry chemical etching or “dry etching” is commonly employed due to itsability to better control the etching process and reduce contaminationlevels. Dry etching effectively etches desired layers through the use ofgases, either by chemical reaction such as using a chemically reactivegas or through physical bombardment, such as plasma etching, using, forexample, argon atoms.

Plasma etching systems have been developed that can effectively etch,for example, silicon, silicon dioxide, silicon nitride, aluminum,tantalum, tantalum compounds, chromium, tungsten, gold, and many othermaterials. Two types of plasma etching reactor systems are in commonuse—the barrel reactor system and the parallel plate reactor system.Both reactor types operate on the same principles and vary primarily inconfiguration only. The typical reactor consists of a vacuum reactorchamber made usually of aluminum, glass, or quartz. A radiofrequency ormicrowave energy source (referred to collectively as RF energy source)is used to activate fluorine-based or chlorine-based gases which act asetchants. Wafers are loaded into the chamber, a pump evacuates thechamber, and the reagent gas is introduced. The RF energy ionizes thegas and forms the etching plasma, which reacts with the wafers to formvolatile products which are pumped away.

Physical etching processes employ physical bombardment. For instance,argon gas atoms may be used to physically bombard a layer to be etched,and a vacuum pump system is used to remove dislocated material. Sputteretching is one physical technique involving ion impact and energytransfer. The wafer to be etched is attached to a negative electrode, or“target,” in a glow-discharge circuit. Positive argon ions bombard thewafer surface, resulting in the dislocation of the surface atoms. Poweris provided by an RF energy source. Ion beam etching and milling arephysical etching processes which use a beam of low-energy ions todislodge material. The ion beam is extracted from an ionized gas (e.g.,argon or argon/oxygen) or plasma, created by an electrical discharge.

Reactive ion etching (RIE) is a combination of chemical and physicaletching. During RIE, a wafer is placed in a chamber with an atmosphereof chemically reactive gas (e.g., CF₄, CCl₄, CHF₃, and many other gases)at a low pressure. An electrical discharge creates an ion plasma with anenergy of a few hundred electron volts. The ions strike the wafersurface vertically, where they react to form volatile species that areremoved by the low pressure in-line vacuum system.

BRIEF SUMMARY

A method of forming a trilayer Josephson junction may be summarized asincluding depositing a superconducting trilayer including a baseelectrode layer, an insulating layer, and a counter electrode layer;depositing a photoresist mask pattern over the superconducting trilayer;etching a pattern into the superconducting trilayer to form at least oneJosephson junction, wherein etching a pattern into the superconductingtrilayer to form at least one Josephson junction includes removing atleast two portions of the counter electrode layer and removing at leasttwo portions of the insulating layer to expose at least two portions ofthe base electrode layer. Removing at least two portions of the counterelectrode layer may include using a combination of SF6, BCl3, and Cl2 toremove at least two portions of the counter electrode layer, andremoving at least two portions of the insulating layer may include usinga combination of SF6, BCl3, and Cl2 to remove at least two portions ofthe insulating layer.

A method of forming a superconducting trilayer may be summarized asincluding depositing a first layer of niobium; depositing a layer ofaluminum oxide over at least a portion of the first layer of niobium viaatomic layer deposition; and depositing a second layer of niobium overat least a portion of the layer of aluminum oxide. The method mayfurther include depositing a layer of aluminum over at least a portionof the first layer of niobium; and depositing the layer of aluminumoxide over at least a portion of the layer of aluminum.

A method of forming a superconducting trilayer within a chamber may besummarized as including depositing a base layer of niobium within thechamber; depositing a layer of aluminum oxide over at least a portion ofthe base layer of niobium within the chamber; filling the chamber withan inert gas to thermalize the base layer of niobium and the aluminumoxide layer; pumping the inert gas out of the chamber; and depositing atop layer of niobium over at least a portion of the aluminum oxide layerwithin the chamber. Filling the chamber with an inert gas may includefiling the chamber with argon.

A method of depositing a protective cap over a Josephson junction may besummarized as including depositing a superconducting trilayer includingan aluminum oxide layer; patterning the superconducting trilayer toexpose at least a portion of the aluminum oxide layer; pre-cleaning theexposed portion of the aluminum oxide layer; and depositing theprotecting cap over the trilayer. Pre-cleaning the exposed portion ofthe aluminum oxide layer may include pre-cleaning the exposed portion ofthe aluminum oxide layer with ions. Pre-cleaning the exposed portion ofthe aluminum oxide layer may include pre-cleaning the exposed portion ofthe aluminum oxide layer via a gentle, anisotropic low pressure etch.

A method of depositing a hybrid dielectric may be summarized asincluding depositing a first dielectric layer comprising a firstdielectric material; depositing a second dielectric layer over at leasta portion of the first dielectric layer, wherein the second dielectriclayer comprises a second dielectric material; and depositing a thirddielectric layer over at least a portion of the second dielectric layer,wherein the third dielectric layer comprises a third dielectricmaterial. Depositing a third dielectric material may include depositinga same type of material as the first dielectric material. Depositing afirst dielectric material may include depositing a non-oxide dielectric,and depositing a second dielectric material may include depositing anoxide dielectric.

A superconducting integrated circuit may be summarized as including afirst superconducting metal layer; a hybrid dielectric layer thatoverlies the first superconducting metal layer, wherein the hybriddielectric layer comprises a first layer of silicon nitride thatdirectly overlies the first superconducting metal layer, a layer ofsilicon dioxide that directly overlies the first layer of siliconnitride, and a second layer of silicon nitride that directly overliesthe layer of silicon dioxide; and a second superconducting metal layerthat overlies the hybrid dielectric layer, wherein the secondsuperconducting metal layer directly overlies the second layer ofsilicon nitride in the hybrid dielectric layer.

A method of fabricating a superconducting integrated circuit may besummarized as including depositing a first dielectric layer; depositinga negative photoresist mask over the first dielectric layer that tracesout a negative pattern of a desired circuit pattern such that thedesired circuit pattern corresponds to regions of the first dielectriclayer that are not directly covered by the negative photoresist mask;etching the desired circuit pattern into the first dielectric layer toproduce open features in the first dielectric layer; depositing a firstsuperconducting metal layer over the first dielectric layer to at leastpartially fill the open features in the first dielectric layer;planarizing the first superconducting metal layer; depositing a seconddielectric layer to produce a desired inner layer dielectric thickness,wherein the inner layer dielectric thickness is controlled by adeposition process; and depositing a second superconducting metal layerabove the second dielectric layer. Depositing a first superconductingmetal layer may include depositing the first superconducting metal layervia electroplating.

A method of fabricating a superconducting integrated circuit may besummarized as including patterning a first superconducting metal layer;depositing a first dielectric layer over the first superconducting metallayer; depositing a first negative photoresist mask over the firstdielectric layer, wherein the first negative photoresist mask provides anegative of a location of at least one via such that the location of theat least one via corresponds to a region of the first dielectric layerthat is not directly covered by the first negative photoresist mask;etching the first dielectric layer to produce at least one holecorresponding to that at least one via, wherein the at least one holeexposes a portion of the first superconducting metal layer; depositing asecond superconducting metal layer over the first dielectric layer to atleast partially fill the at least one hole and provide a first portionof at least a first via; planarizing the second superconducting metallayer; depositing a second dielectric layer; depositing a secondnegative photoresist mask over the second dielectric layer that tracesout a negative of a desired circuit pattern such that the desiredcircuit pattern corresponds to regions of the second dielectric layerthat are not directly covered by the second negative photoresist mask;etching the desired circuit pattern into the second dielectric layer toproduce open features in the second dielectric layer; depositing a thirdnegative photoresist mask over the second dielectric layer, wherein thethird negative photoresist mask provides a negative of a location of atleast one via such that the location of the at least one via correspondsto a region of the second dielectric layer that is not directly coveredby the third negative photoresist mask, and the location of the at leastone via is within an open feature in the second dielectric layer;etching the second dielectric layer to produce at least one holecorresponding to the at least one via, wherein the at least one holeexposes a portion of the first portion of the first via; depositing athird superconducting metal layer over the second dielectric layer to atleast partially fill the at least one hole in the second dielectriclayer and provide a second portion of the first via and to at leastpartially fill the open features in the second dielectric layer; andplanarizing the third superconducting metal layer. At least one ofdepositing a second superconducting metal and depositing a thirdsuperconducting metal layer may include electroplating. At least one ofplanarizing the second superconducting metal layer and planarizing thethird superconducting metal layer may include chemical mechanicalplanarization.

A substrate for use in a superconducting integrated circuit may besummarized as including a base layer comprising silicon; and a top layercomprising aluminum oxide. The base layer may include at least one of:undoped silicon, doped silicon, sapphire, and quartz. The base layer maybe thicker than the top layer.

A method of depositing a superconducting metal layer in an integratedcircuit may be summarized as including depositing a first portion of thesuperconducting metal layer; stopping the depositing of the firstportion of the superconducting metal layer to prevent excessive heating;cooling the superconducting metal layer; and depositing a second portionof the superconducting metal layer over the first portion of thesuperconducting metal layer. The method may further include stopping thedepositing of the second portion of the superconducting metal layer toprevent excessive heating; cooling the superconducting metal layer; anddepositing a third portion of the superconducting metal layer over thesecond portion of the superconducting metal layer.

A method of aligning multiple layers in a multilayered superconductingintegrated circuit may be summarized as including patterning a firstsuperconducting metal layer to include at least one alignment mark;depositing a first dielectric layer over the first superconducting metallayer; patterning the first dielectric layer to expose the at least onealignment mark; depositing a second superconducting metal layer over thefirst dielectric layer such that an impression of the at least onealignment mark is formed on an exposed surface of the secondsuperconducting metal layer; and aligning a photoresist mask to theimpression of the at least one alignment mark on the secondsuperconducting metal layer. The method may further include depositingthe photoresist mask over the second superconducting metal layer.

A method of fabricating a superconducting integrated circuit may besummarized as including depositing a first superconducting metal layer;depositing a superconducting protective capping layer over the firstsuperconducting metal layer; patterning both the first superconductingmetal layer and the superconducting protective capping layer over thefirst superconducting metal layer; depositing a dielectric layer overthe patterned superconducting protective capping layer; etching a holethrough the dielectric layer to expose a portion of at least one of thesuperconducting protective capping layer or the first superconductingmetal layer; and depositing a second superconducting metal layer overthe dielectric layer such that at least a portion of the secondsuperconducting metal layer at least partially fills the hole throughthe dielectric layer and forms a superconducting electrical connectionwith at least one of the superconducting protective capping layer or thefirst superconducting metal layer. Depositing a superconductingprotective capping layer over the first superconducting metal layer mayinclude depositing a titanium nitride layer over the firstsuperconducting metal layer.

A superconducting integrated circuit may be summarized as including afirst patterned superconducting metal layer; a superconductingprotective capping layer positioned over the first patternedsuperconducting metal layer, wherein the superconducting protectivecapping layer is patterned to match a pattern in the first patternedsuperconducting metal layer; a dielectric layer positioned over thesuperconducting protective capping layer; a second patternedsuperconducting metal layer positioned over the dielectric layer; and asuperconducting via that extends through the dielectric layer andsuperconductingly electrically couples a portion of the second patternedsuperconducting metal layer to at least one of a portion of thesuperconducting protective capping layer or a portion of the firstsuperconducting metal layer. The superconducting protective cappinglayer may include titanium nitride.

A method of fabricating a Josephson junction pentalayer may besummarized as including depositing a first superconducting metal layer;depositing a first insulating barrier over the first superconductingmetal layer, wherein the first insulating barrier has a first thickness;depositing a second superconducting metal layer over the firstinsulating barrier; depositing a second insulating barrier over thesecond superconducting metal layer, wherein the second insulatingbarrier has a second thickness that is different from the firstthickness of the first insulating barrier; and depositing a thirdsuperconducting metal layer over the second insulating barrier.Depositing a second insulating barrier over the second superconductingmetal layer, wherein the second insulating barrier has a secondthickness that is different from the first thickness of the firstinsulating barrier, may include depositing a second insulating barrierover the second superconducting metal layer, wherein the secondinsulating barrier has a second thickness that is larger than the firstthickness of the first insulating barrier.

A superconducting integrated circuit may be summarized as including aJosephson junction pentalayer including a first superconducting metallayer; a first insulating barrier having a first thickness, wherein thefirst insulating barrier is positioned over the first superconductingmetal layer; a second superconducting metal layer positioned over thefirst insulating barrier; a second insulating barrier having a secondthickness, wherein the second insulating barrier is positioned over thesecond superconducting metal layer; and a third superconducting metallayer positioned over the second insulating barrier; a dielectric layerpositioned over the Josephson junction pentalayer; a superconductingwiring layer positioned over the dielectric layer; and at least onesuperconducting via that superconductingly electrically couples at leasta portion of the superconducting wiring layer to at least a portion ofthe Josephson junction pentalayer. The second thickness of the secondinsulating barrier may be greater than the first thickness of the firstinsulating barrier. At least a first portion of the Josephson junctionpentalayer may be patterned to form a first Josephson junctionincluding: a first portion of the third superconducting metal layer; afirst portion of the second insulating barrier; a first portion of thesecond superconducting metal layer; a first portion of the firstinsulating barrier; and a first portion of the first superconductingmetal layer, and at least one superconducting via may superconductinglyelectrically couple a first portion of the second superconducting wiringlayer to the first portion of the third superconducting metal layer. Atleast a second portion of the Josephson junction pentalayer may bepatterned to form a second Josephson junction including: a secondportion of the second superconducting metal layer; a second portion ofthe first insulating barrier; and a second portion of the firstsuperconducting metal layer, and at least one superconducting via maysuperconductingly electrically couple a second portion of the secondsuperconducting wiring layer to the second portion of the secondsuperconducting metal layer. At least a first portion of the Josephsonjunction pentalayer may be patterned to form a first Josephson junctionincluding: a first portion of the second superconducting metal layer; afirst portion of the first insulating barrier; and a first portion ofthe first superconducting metal layer, and at least one superconductingvia may superconductingly electrically couple a first portion of thesecond superconducting wiring layer to the first portion of the secondsuperconducting metal layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elementsor acts. The sizes and relative positions of elements in the drawingsare not necessarily drawn to scale. For example, the shapes of variouselements and angles are not drawn to scale, and some of these elementsare arbitrarily enlarged and positioned to improve drawing legibility.Further, the particular shapes of the elements as drawn, are notintended to convey any information regarding the actual shape of theparticular elements, and have been solely selected for ease ofrecognition in the drawings.

FIG. 1A is an elevational, partially sectioned, view of a portion of asuperconducting integrated circuit including an unpatterned trilayer,according to one illustrated embodiment.

FIG. 1B is an elevational, partially sectioned, view of the portion of asuperconducting integrated circuit of FIG. 1A after further processingoperation(s) and which includes a patterned trilayer and a counterelectrode, according to one illustrated embodiment.

FIG. 1C is an elevational, partially sectioned, view of a portion of asuperconducting integrated circuit of FIG. 1B after further processingoperation(s) and which includes individual Josephson junctions,according to one illustrated embodiment.

FIG. 2 is a flow diagram showing a method of fabricating a Josephsonjunction from a niobium/aluminum oxide/niobium trilayer, according toone illustrated embodiment.

FIG. 3 is a flow diagram showing a method of fabricating aniobium/aluminum oxide/niobium trilayer, according to one illustratedembodiment.

FIG. 4 is a flow diagram showing a method of forming a niobium/aluminumoxide/niobium trilayer, according to one illustrated embodiment.

FIG. 5 is an elevational, partially sectioned, view of a portion of asuperconducting integrated circuit including a Josephson junctioncovered by a protective cap, according to one illustrated embodiment.

FIG. 6 is a flow diagram showing a method of depositing a protective capover a trilayer Josephson junction, according to one illustratedembodiment.

FIG. 7 is an elevational, partially sectioned, view of a portion of asuperconducting integrated circuit including hybrid dielectric layers,according to one illustrated embodiment.

FIG. 8 is a flow diagram showing a method of depositing a hybriddielectric, according to one illustrated embodiment.

FIG. 9A is an elevational, partially sectioned, view of a portion of asuperconducting integrated circuit during a masking stage of asubtractive patterning process, according to one illustrated embodiment.

FIG. 9B is an elevational, partially sectioned, view of a portion of thesuperconducting integrated circuit of FIG. 9A after an etching stage ofthe subtractive patterning process, according to one illustratedembodiment.

FIG. 9C is an elevational, partially sectioned, view of a portion of thesuperconducting integrated circuit of FIG. 9B after a dielectricdeposition stage of the subtractive patterning process, according to oneillustrated embodiment.

FIG. 9D is an elevational, partially sectioned, view of a portion of thesuperconducting integrated circuit of FIG. 9C after a dielectricplanarization stage of the subtractive patterning process, according toone illustrated embodiment.

FIG. 9E is an elevational, partially sectioned, view of a portion of thesuperconducting integrated circuit of FIG. 9D after a secondsuperconducting metal layer has been deposited, according to oneillustrated embodiment.

FIG. 10A is an elevational, partially sectioned, view of a portion of asuperconducting integrated circuit during a masking stage of an additivepatterning process, according to one illustrated embodiment.

FIG. 10B is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 10A after an etchingstage of an additive patterning process, according to one illustratedembodiment.

FIG. 10C is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 10B after a metaldeposition stage of an additive patterning process, according to oneillustrated embodiment.

FIG. 10D is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 10C after a metalplanarization stage of an additive patterning process, according to oneillustrated embodiment.

FIG. 10E is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 10D after a dielectriclayer has been deposited, according to one illustrated embodiment.

FIG. 10F is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 10E after an etch-stoplayer has been deposited, according to one illustrated embodiment.

FIG. 10G is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 10F after a dielectriclayer has been deposited, according to one illustrated embodiment.

FIG. 10H is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 10G after a negativephotoresist mask has been deposited over the dielectric layer, accordingto one illustrated embodiment.

FIG. 10I is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 10H after an etchingstage of an additive patterning process, according to one illustratedembodiment.

FIG. 10J is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 10I after a metaldeposition stage of an additive patterning process, according to oneillustrated embodiment.

FIG. 10K is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 10J after a metalplanarization stage of an additive patterning process, according to oneillustrated embodiment.

FIG. 11 is a flow diagram showing a method of employing an additivepatterning technique in a superconducting integrated circuit fabricationprocess to realize improved ILD thickness control, according to oneillustrated embodiment.

FIG. 12A is an elevational, partially sectioned, view of a portion of asuperconducting integrated circuit during a via masking stage of asuperconducting dual Damascene process, according to one illustratedembodiment.

FIG. 12B is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 12A after dielectricetching, metal deposition, and metal planarization/polishing stages ofan additive patterning process, according to one illustrated embodiment.

FIG. 12C is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 12B during a wiringmasking stage of a dual Damascene process after etch stop layers anddielectric layer have been deposited, according to one illustratedembodiment.

FIG. 12D is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 12C after a wiringetching stage of an additive patterning process, according to oneillustrated embodiment.

FIG. 12E is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 12D during a via maskingstage of a dual Damascene process, according to one illustratedembodiment.

FIG. 12F is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 12E after a via etchingstage of a dual Damascene process, according to one illustratedembodiment.

FIG. 12G is an elevational, partially sectioned, view of the portion ofthe superconducting integrated circuit of FIG. 12F after superconductingmetal deposition and planarization has been completed, according to oneillustrated embodiment.

FIG. 13 is a flow diagram showing a method of implementing asuperconducting dual Damascene process, according to one illustratedembodiment.

FIG. 14 is an elevational, partially sectioned, view of a portion of asubstrate for use in a superconducting integrated circuit, according toone illustrated embodiment.

FIG. 15 is a flow diagram showing a method of performing multi-stagedmetal deposition, according to one illustrated embodiment.

FIG. 16 is an elevational, partially sectioned, view of a portion of anintegrated circuit showing an impression of an alignment mark in asuperconducting metal layer, according to one illustrated embodiment.

FIG. 17 is a flow diagram showing a method of aligning multiple layersin a multi-layered superconducting integrated circuit without using anopen frame and match technique, according to one illustrated embodiment.

FIG. 18 is a sectional view of a portion of an exemplary superconductingintegrated circuit showing a superconducting via having non-verticalsidewalls, according to one illustrated embodiment.

FIG. 19 shows a method for forming a superconducting via in accordancewith the present systems and methods.

FIG. 20 is a sectional view of a portion of a superconducting integratedcircuit including a superconducting protective capping layer over asuperconducting metal layer in accordance with the present systems andmethods.

FIG. 21A is a sectional view of a portion of a superconductingintegrated circuit including a Josephson junction pentalayer inaccordance with the present systems and methods.

FIG. 21B is a sectional view of a portion of an exemplarysuperconducting integrated circuit in accordance with the presentsystems and methods.

FIG. 22 shows a method for forming a Josephson junction pentalayer inaccordance with the present systems and methods.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various disclosedembodiments. However, one skilled in the relevant art will recognizethat embodiments may be practiced without one or more of these specificdetails, or with other methods, components, materials, etc. In otherinstances, well-known structures associated with superconductivecircuits or structures, quantum computer circuits or structures and/orcryogenic cooling systems such as dilution refrigerators have not beenshown or described in detail to avoid unnecessarily obscuringdescriptions of the embodiments.

Unless the context requires otherwise, throughout the specification andclaims which follow, the word “comprise” and variations thereof, suchas, “comprises” and “comprising” are to be construed in an open,inclusive sense, that is as “including, but not limited to.”

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its sense including “and/or” unless the contentclearly dictates otherwise.

As used in this specification and the appended claims the terms “carriedby,” “carried on,” or variants thereof, and similarly the terms “over”and “above,” mean that one structure is directly or indirectly supportedin at least some instances by another structure, for example directly ona surface thereof, spaced above or below a surface thereof by one ormore intervening layers or structures or located therein.

The headings and Abstract of the Disclosure provided herein are forconvenience only and do not interpret the scope or meaning of theembodiments.

Unless the specific context requires otherwise, throughout thisspecification the terms “deposit,” “deposited,” “deposition,” and thelike are generally used to encompass any method of material deposition,including but not limited to physical vapor deposition (PVD), chemicalvapor deposition (CVD), plasma-enhanced PVD, plasma-enhanced CVD, andatomic layer deposition (ALD).

The various embodiments described herein provide systems and methods forfabricating superconducting integrated circuits. As previouslydescribed, in the art superconducting integrated circuits tend to befabricated in research environments outside of state-of-the-artsemiconductor fabrication facilities, even though superconductingintegrated circuits are typically fabricated using many of the sametools and techniques that are traditionally used in the semiconductorfabrication industry. Due to issues unique to superconducting circuits,semiconductor processes and techniques generally need to be modified foruse in superconductor chip and circuit fabrication. Such modificationstypically are not obvious and may require some experimentation.

A Josephson junction is a common element in superconducting integratedcircuits. Physically, a Josephson junction is a small interruption in anotherwise continuous superconducting current path, typically realized bya thin insulating barrier sandwiched in between two superconductingelectrodes. In superconducting integrated circuits, Josephson junctionsare typically fabricated as a stack comprising a superconducting baseelectrode overlaid with a thin insulating layer, which is then overlaidwith a superconducting counter electrode. Thus, a Josephson junction isusually formed as a three-layer, or “trilayer,” structure. A trilayermay be deposited completely over an entire wafer (i.e., in the same waythat metal wiring and dielectric layers are deposited) and thenpatterned to define individual Josephson junctions.

FIG. 1A shows a sectional view of a portion of a superconductingintegrated circuit 100 a including an unpatterned trilayer 110. Trilayer110 is carried on a substrate 130 and comprises: a superconducting baseelectrode 111 formed of, for example, niobium Nb; an insulating barrier112 formed of, for example, an aluminum oxide AlOx; and asuperconducting counter electrode 113 formed of, for example, niobiumNb. Substrate 130 may comprise silicon, sapphire, quartz, silicondioxide, or any similar suitable material. In some embodiments, theupper surface of a niobium base electrode 111 may be covered with a thinlayer of aluminum (not illustrated) upon which aluminum oxide layer 112is grown (thus, a “trilayer” may in fact comprise four layers: a niobiumbase electrode, a layer of aluminum, a layer of aluminum oxide grownupon the layer of aluminum, and a niobium counter electrode). Trilayer110 may be patterned by, for example, a lithographic photoresist maskingand plasma etching process to form individual Josephson junctions. Insome applications, counter electrode 113 may be patterned to defineindividual junctions and base electrode 111 may be used as a wiringlayer providing electrical connections between junctions. During thepatterning of counter electrode 113, aluminum oxide layer 112 may beused as an etch-stop and the niobium of counter electrode 113 may beetched using a chemistry that does not etch through aluminum oxide layer112.

FIG. 1B shows a sectional view of a portion of a superconductingintegrated circuit 100 b including a patterned trilayer 110. FIG. 1Bdepicts superconducting integrated circuit 100 a from FIG. 1A aftercounter electrode 113 has been etched to define individual Josephsonjunctions 121 and 122 while using aluminum oxide layer 112 as anetch-stop. Dielectric layer 140 (which may comprise, e.g., silicondioxide) has also been deposited over trilayer 110. When aluminum oxidelayer 112 is used as an etch-stop, the regions of aluminum oxide layer112 that are outside of the individual Josephson junctions 121 and 122may be left in place (i.e., unetched). However, Applicants haverecognized that in applications where Josephson junctions 121 and 122and/or the superconducting wiring in base electrode 111 are particularlysensitive to noise (e.g., in applications employing superconductingqubits such as in a superconducting quantum processor), the interfacebetween niobium base electrode 111 and aluminum oxide layer 112 and/orthe interface between aluminum oxide layer 112 and dielectric layer 140may introduce unwanted and unnecessary noise into the system. Inaccordance with the present systems and methods, such noise may beavoided by removing the regions of aluminum oxide layer 112 that areoutside of the individual Josephson junctions 121 and 122 and therebyreducing the total number of material interfaces in the integratedcircuit structure.

FIG. 1C shows a sectional view of a portion of a superconductingintegrated circuit 100 c including a patterned trilayer 110 from whichexcess aluminum oxide 112 has been removed in accordance with thepresent systems and methods. FIG. 1C depicts superconducting integratedcircuit 100 a from FIG. 1A after counter electrode 113 has been etchedto define individual Josephson junctions 121 and 122 without usingaluminum oxide layer 112 as an etch-stop. Thus, integrated circuit 100 cdiffers from integrated circuit 100 b in that regions of aluminum oxidelayer 112 that are outside of the individual Josephson junctions 121 and122 have been etched away in integrated circuit 100 c. Regions ofaluminum oxide layer 112 that are outside of the individual Josephsonjunctions 121 and 122 may be etched away, for example, during thepatterning and etching of niobium counter electrode 113 by employing anetching chemistry that does not use aluminum oxide layer 112 as anetch-stop. SF₆ may be used to etch niobium and a combination of BCl₃,Cl₂, and N₂ may be used to etch aluminum. In accordance with the presentsystems and methods, niobium counter electrode 113 may be etched using acombination of SF₆, BCl₃, Cl₂, and/or N₂ because such an etch chemistrymay also remove the regions of aluminum oxide layer 112 that are outsideof the individual Josephson junctions 121 and 122. Removing aluminumoxide during the etch of individual Josephson junctions may reduce thenumber of material interfaces in a superconducting integrated circuitand, as a result, reduce noise that may otherwise have an adverse effecton circuit performance. In accordance with the present systems andmethods, regions of aluminum oxide layer 112 that are outside of theindividual Josephson junctions 121 and 122 may also be removed in aseparate process act or operation after the Josephson junction 121 and122 have been defined by employing a separate etch specifically designedto remove aluminum oxide.

Furthermore, in accordance with the present systems and methods, anysuperconducting fabrication process that involves etching niobium (evenif the process is not to pattern Josephson junctions and/or removealuminum oxide layers) may benefit from a modified niobium-etchingchemistry that employs a combination of SF₆ with BCl₃, Cl₂, and/or N₂because such may result in a smoother, flatter niobium surface profilecompared to a SF₆ etch on its own, and particularly smoother, flatterniobium sidewalls.

The process of removing excess aluminum oxide during Josephson junctionfabrication (i.e., in going from FIG. 1A to FIG. 1C) is summarized inFIG. 2. FIG. 2 shows a method 200 of fabricating a Josephson junctionfrom a niobium/aluminum oxide/niobium trilayer in accordance with thepresent systems and methods. Method 200 includes three acts 201-203,though those of skill in the art will appreciate that in alternativeembodiments certain acts may be omitted and/or additional acts may beadded. Those of skill in the art will appreciate that the illustratedorder of the acts is shown for exemplary purposes only and may change inalternative embodiments. At 201, an Nb-AlOx-Nb trilayer is deposited,e.g., employing techniques described previously. At 202, a photoresistmask pattern is deposited over or on top of the trilayer. Thephotoresist mask layer may cover some portions of the trilayer and leaveother portions of the trilayer uncovered. Those portions of the trilayerthat are uncovered will be etched away during the etch process (see act203), while those portions of the trilayer that are covered will remainafter the etch process. At 203, the pattern defined by the photoresistmask is etched into the trilayer to form at least one Josephsonjunction. Where a Josephson junction is formed, a portion of aluminumoxide will be sandwiched in between two portions of niobium metal (i.e.,a patterned counter electrode over a base electrode). Where no Josephsonjunction is formed, the niobium counter electrode layer of the trilayeris etched away (i.e., stripped) and at least the portion of aluminumoxide that underlies the stripped counter electrode is also etched awaysuch that any excess aluminum oxide that is not part of at least oneJosephson junction is removed.

In processes that employ niobium etching, the photoresist mask typicallyneeds to be removed after the niobium etching has been completed. Theetching chemistry that is used to etch the niobium cannot, by design,etch the photoresist mask or the process would fail to pattern theniobium. However, the photoresist mask does typically need to be removedonce the niobium etching is complete in order to, for example, allow viaconnections to be made to subsequent niobium layers added to the circuit(e.g., additional niobium layers carried on the etched niobium layer).In the art, the photoresist mask (and related polymers that may beformed by interactions between the photoresist mask and the niobiumitself) is typically stripped away via an O₂ plasma etching/bombardmentprocess. However, an O₂ plasma on its own may not be sufficient toremove some of the polymers that result from the adhesion of thephotoresist mask to the niobium metal. In accordance with the presentsystems and methods, a modified photoresist-stripping process may employa combination of CF₄ and O₂ plasma to more reliably remove photoresistmask residue (e.g., polymers formed by interactions between thephotoresist mask and the niobium metal) from the surface of niobiummetal.

In the fabrication of a Nb-AlOx-Nb trilayer, a first layer of niobiummay be deposited and a thin layer of aluminum may be deposited over thefirst layer of niobium. The aluminum is then exposed to O₂ gas to grow alayer of aluminum oxide on the upper surface of the aluminum. It istypically desired to produce a very specific and uniform aluminum oxidethickness at this stage. The thickness of the AlOx layer ultimatelyaffects the critical current of any Josephson junctions in the resultingsuperconducting integrated circuit and is therefore an importantfabrication parameter. In this process, the thickness of the AlOx layeris determined by several parameters, including the duration of theexposure to the O₂ gas, the concentration of the O₂ gas, thetemperature, the pressure, etc. Thus, given the O₂ concentration, thepressure, the temperature, etc., an O₂ exposure time is calculated toproduce the desired AlOx thickness. Once the calculated O₂ exposure timehas elapsed, a second layer of niobium is deposited over the aluminumoxide layer to complete the Nb-AlOx-Nb trilayer. This process forfabricating Nb-AlOx-Nb trilayers is well-established, but ultimatelyprovides limited control of the thickness of the AlOx layer produced.The AlOx thickness is determined indirectly through a calculationinvolving many inputs, and each of these inputs carries some uncertaintywhich affects the resulting thickness of AlOx produced. For example, anyvariation in the pressure, temperature, O₂ concentration, O₂ exposuretime, etc. will impact the thickness of the AlOx layer produced.Accordingly, there remains a need in the art for a method of fabricatingNb-AlOx-Nb trilayers that achieves improved AlOx thickness control.

In accordance with the present systems and methods, improved AlOxthickness control in the fabrication of Nb-AlOx-Nb trilayers may beachieved by directly depositing the aluminum oxide layer via atomiclayer deposition. FIG. 3 shows a method 300 for fabricating aniobium/aluminum oxide/niobium trilayer in accordance with the presentsystems and methods. Method 300 includes three acts 301-303, thoughthose of skill in the art will appreciate that in alternativeembodiments certain acts may be omitted and/or additional acts may beadded. Those of skill in the art will appreciate that the illustratedorder of the acts is shown for exemplary purposes only and may change inalternative embodiments. At 301, a first layer of niobium is depositedvia a standard deposition process such as chemical vapor deposition,physical vapor deposition, or similar. The niobium may be deposited overa substrate or over any other layer of an integrated circuit (such asover a dielectric layer, or over another metal layer). The upper surfaceof the niobium is preferably smooth and substantially uniform. If thedesired smoothness cannot be achieved during the deposition processalone then the upper surface of the niobium may be planarized and/orpolished via a chemical-mechanical planarization process (e.g., CMP). At302, a layer of aluminum oxide is deposited over the smooth uppersurface of the first niobium layer via atomic layer deposition. Atomiclayer deposition allows the aluminum oxide layer to be actively builtand may enable improved control of the thickness of the aluminum oxidelayer compared to the O₂ exposure process known in the art and describedabove. In some embodiments, the adhesion of the aluminum oxide layer tothe niobium layer may be increased by first depositing a thin aluminumlayer (e.g., via CVD, PVD, or ALD) over the smooth upper surface of theniobium layer and then depositing the aluminum oxide layer via atomiclayer deposition over the thin aluminum layer (such a thin aluminumlayer may be planarized or polished to improve smoothness, ifnecessary). At 303, a second layer of niobium is deposited over thealuminum oxide layer via a standard deposition process (e.g., CVD orPVD). The deposition of the second niobium layer completes theNb-AlOx-Nb trilayer which may be used to form one or more Josephsonjunctions in a superconducting integrated circuit.

Trilayer deposition (and specifically the aluminum oxidedeposition/growth process) is particularly sensitive to temperature. Theexistence of a non-uniform temperature (e.g., a temperature gradient)across a wafer may result in non-uniform aluminum oxide thickness acrossthe wafer. Such a non-uniform temperature can result during a heatingprocess and/or a cooling process alike. For example, a wafer may beheated during an aluminum oxide deposition process and may cool beforethe subsequent niobium deposition process. During this cooling, thealuminum oxide layer may continue to form and grow. It is thereforedesirable to ensure substantially uniform cooling of the wafer inbetween the aluminum oxide deposition/growth and the subsequent niobiumdeposition of a trilayer fabrication process. In accordance with thepresent systems and methods, uniformity during such cooling may beenhanced by filling the deposition chamber with an inert gas (e.g.,argon) to provide a thermalization medium having a substantially uniformpressure across the wafer. In some embodiments, trilayers may bedeposited on multiple wafers in the same chamber simultaneously andfilling the chamber with an inert cooling gas (e.g., argon) may improveuniformity of temperature across multiple wafers.

FIG. 4 shows a method 400 for forming a niobium/aluminum oxide/niobiumtrilayer in accordance with the present systems and methods. Method 400includes four acts 401-404, though those of skill in the art willappreciate that in alternative embodiments certain acts may be omittedand/or additional acts may be added. Those of skill in the art willappreciate that the illustrated order of the acts is shown for exemplarypurposes only and may change in alternative embodiments. At 401, a baselayer of niobium is deposited over a wafer. The niobium may be depositedvia any known deposition technique, including CVD, PVD, ALD, and thelike. The deposition may be carried out in a sealed chamber. At 402, analuminum oxide layer is deposited over the niobium base layer. In someembodiments, “depositing an aluminum oxide layer” may include depositinga thin layer of aluminum directly on the niobium base layer and thengrowing an aluminum oxide layer on the thin layer of aluminum (e.g., byexposing the aluminum layer to oxygen gas). At 403, the chamber isfilled with a substantially uniform pressure of an inert gas (such as,for example, argon). The inert gas provides a medium through whichthermal energy may be dissipated and ensures the wafer (and inparticular, the aluminum oxide layer located on the surface of thewafer) has a substantially uniform temperature as the aluminum oxidelayer cools. At 404, the inert gas is pumped out of the chamber and thetop layer of niobium is deposited over the aluminum oxide layer. Theuniform cooling process of act 403 may improve aluminum oxide thicknessuniformity across the wafer.

A Josephson junction may be formed in a Nb-AlOx-Nb trilayer bypatterning the counter electrode as described in FIGS. 1A to 1C. USPatent Publication 2011-0089405 (which is incorporated herein byreference in its entirety) further describes protecting a formedJosephson junction from subsequent processing acts by depositing a cap(formed of, e.g., silicon nitride SiN) over top of the Josephsonjunction counter electrode. FIG. 5 shows a sectional view of a portionof a superconducting integrated circuit 500 including a Josephsonjunction 510 covered by a protective cap 520. As described in US PatentPublication 2011-0089405, cap 520 may be formed of, for example, siliconnitride, hydrogenated amorphous silicon, an organic polymer dielectricmaterial or a similar dielectric material. Josephson junction 510includes base electrode 511 (formed of superconducting metal, such asniobium), insulating barrier 512 (formed of, e.g., aluminum oxide), andcounter electrode 513 (formed of superconducting metal, such asniobium). A challenge in depositing cap 520 in superconductingintegrated circuit 500 is that the cap material may not adhere very wellto aluminum oxide layer 512. This challenge may be overcome by etchingaway excess aluminum oxide as described previously and illustrated inFIG. 1C (compared to FIG. 1B). However, in circuits where removingexcess aluminum oxide is not practical, adhesion between cap 520 andaluminum oxide layer 512 may be improved by pre-cleaning the uppersurface of aluminum oxide layer 512 to, among other things, removemoisture and any other particles that may contaminate the exposedsurface of aluminum oxide layer 512. This pre-cleaning may include, forexample, battering the exposed surface of aluminum oxide layer 512 withions and/or employing a gentle, anisotropic low pressure etch.

FIG. 6 shows a method 600 for depositing a protective cap over atrilayer Josephson junction in accordance with the present systems andmethods. Method 600 includes four acts 601-604, though those of skill inthe art will appreciate that in alternative embodiments certain acts maybe omitted and/or additional acts may be added. Those of skill in theart will appreciate that the illustrated order of the acts is shown forexemplary purposes only and may change in alternative embodiments. At601, a trilayer is deposited (e.g., over a wafer, or over a surface of adielectric layer, or over a surface of a metal layer, etc.) as describedpreviously. The trilayer may include, for example, a Nb/AlOx/Nbtrilayer. At 602, the trilayer is patterned by, for example, alithographic process as described previously. At 603, the exposedsurface of the patterned trilayer (e.g., the upper surface) ispre-cleaned in accordance with the present systems and methods. Theexposed surface of the trilayer may include both superconducting metalsurfaces (i.e., niobium counter electrode surfaces) and insulatingbarrier surfaces (i.e., aluminum oxide surfaces). As previouslydescribed, this pre-cleaning may include, for example, battering theexposed surfaces of the aluminum oxide layer with ions and/or employinga gentle, anisotropic low pressure etch. At 604, a protective cap isdeposited over the trilayer. The cap may include, for example, siliconnitride and may help to protect the trilayer (and in particular, thealuminum oxide layer) from being degraded in subsequent processing. Thepre-cleaning at 603 may improve adhesion between the cap and thealuminum oxide layer.

Cap 520 described above comprises a layer of material (e.g., SiN) thatoverlies a Josephson junction (510) in order to protect the Josephsonjunction (and especially the aluminum oxide layer 512) from subsequentprocessing operations. In accordance with the present systems andmethods, a similar “capping” technique may be used to reduce noise insuperconducting integrated circuits by shielding wiring layers fromoxides that may be present in dielectric layers (e.g., silicon dioxide)and/or prevent wiring layers from oxidizing during the deposition ofoxide dielectric layers (e.g., silicon dioxide). For example, hybriddielectrics may be employed to effectively sandwich metal wiring layersbetween non-oxide caps (such as SiN) both above and below dielectriclayers.

FIG. 7 is a sectional view of a portion of a superconducting integratedcircuit 700 including hybrid dielectric layers 710 and 720 in accordancewith the present systems and methods. Superconducting integrated circuit700 includes metal wiring layers 730 and 740, each of which includes apatterned conductor formed of a superconducting material, such asniobium or aluminum. Hybrid dielectric layer 710 is itself comprised ofthree layers: a base layer of non-oxide dielectric material 711 (e.g.,SiN), a layer of silicon dioxide 712, and a top layer of non-oxidedielectric material 713 (e.g., SiN). Hybrid dielectric layer 720 issimilarly comprised of three layers: a base layer of non-oxidedielectric material 721 (e.g., SiN), a layer of silicon dioxide 722, anda top layer of non-oxide dielectric material 723 (e.g., SiN). Non-oxidedielectric layer 711 shields metal wiring layer 730 from silicon dioxidelayer 712. Similarly, non-oxide dielectric layers 713 and 721 shieldmetal wiring layer 740 from silicon dioxide layers 712 and 722,respectively. Thus, hybrid dielectric layers 710 and 720 enableisolation of metal wiring layers 730 and 740 from the oxides present insilicon dioxide layers 712 and 722 and may therefore help to reducenoise in superconducting integrated circuit 700. Similarly, hybriddielectric layers 710 and 720 help to prevent oxidation of metal wiringlayers 730 and 740, respectively, during deposition of silicon dioxidelayers 712 and 722. Those of skill in the art will appreciate that thecircuit details of integrated circuit 700 are illustrative only andsimilar hybrid dielectric processing may be employed in asuperconducting integrated circuit that includes Josephson junctions(e.g., trilayers) and/or via connections between metal wiring layers.

FIG. 8 shows a method 800 for depositing a hybrid dielectric inaccordance with the present systems and methods. Method 800 includesthree acts 801-803, though those of skill in the art will appreciatethat in alternative embodiments certain acts may be omitted and/oradditional acts may be added. Those of skill in the art will appreciatethat the illustrated order of the acts is shown for exemplary purposesonly and may change in alternative embodiments. At 801, a first layercomprising a first dielectric material is deposited. The firstdielectric material may include a non-oxide dielectric, such as siliconnitride and the first layer may be deposited by any deposition process,including CVD, PVD, and/or ALD. The first layer may be deposited, forexample, on top of or over a metal layer in an integrated circuit. At802, a second layer comprising a second dielectric material may bedeposited on top of or over the first layer. The second dielectricmaterial may include an oxide dielectric, such as silicon dioxide andthe second layer may be deposited by any deposition process, includingCVD, PVD, and/or ALD. At 803, a third layer comprising the firstdielectric material may be deposited on top of or over the second layer.In some embodiments, the third layer may comprise a third dielectricmaterial that is a non-oxide dielectric. In some embodiments, at leastone layer may be polished or planarized after being deposited, beforeanother layer is deposited thereon. For example, the first layer may bepolished or planarized before the second layer is deposited thereon. Insome embodiments, a metal layer (e.g., a superconducting metal layer)may be deposited over the third layer. If the integrated circuitincludes additional metal layers, then each metal layer may be separatedfrom upper and/or lower metal layers by a respective hybrid dielectricformed by method 800.

In the semiconductor industry, a process known as “additive patterning”or “Damascene” processing has been developed to process materials thatcannot be directly patterned by standard photoresist masking and plasmaetching techniques. For example, semiconductor integrated circuits thatemploy copper interconnections (as opposed to, for example, aluminuminterconnections) are typically fabricated by this additive patterningapproach because copper is incompatible with standard photoresistmasking and plasma etching techniques. Copper may be preferable toaluminum in some semiconducting applications because copper is a betterconductor than aluminum, meaning that copper circuits use less energyand can include smaller components.

In additive patterning, the underlying dielectric layer is patternedwith open features (e.g., trenches) and then a thick layer of theconductor is deposited over the dielectric such that it completely fillsthe open features of the pattern. Chemical-mechanicalplanarization/polishing (CMP) is then employed to remove the excessconductor down to the level of the top of the underlying dielectric. Theresult is a patterned conductor produced by filling in a pattern in thedielectric as opposed to the more traditional approach of etching apattern directly into the conductor itself. In other words, “additivepatterning” is a process whereby a conductor is added to an existingpattern. Conversely, standard photoresist masking and plasma etchingtechniques provide “subtractive patterning” whereby portions of aconductor are subtracted (i.e., etched) away to produce a pattern.

As described above, additive patterning is used in the semiconductorindustry in order to pattern materials (e.g., copper) that are notcompatible with standard photoresist masking and plasma etchingtechniques. In accordance with the present systems and methods, atechnique that is similar in some respects may be employed in thefabrication of superconducting integrated circuits, albeit motivated bycompletely different reasons than those of the semiconductor industry.

In a multilayered integrated circuit (either semiconducting orsuperconducting), successive layers of conductive wiring are typicallyseparated from one another by inner layer dielectrics (“ILDs”). ILDsprovide structural support for the whole circuit while electricallyinsulating adjacent conductive layers. The thickness of an ILDdetermines the distance between two adjacent conductive layers in thecircuit, and this distance influences, among other things, inductive andcapacitive coupling between the adjacent conductive layers. Insemiconducting integrated circuits, inductive and capacitive couplingbetween adjacent conductive layers are typically not crucial designfeatures. Conversely, in superconducting integrated circuits inductiveand/or capacitive coupling between conductive layers can be crucialfeatures of the circuit design. Superconducting integrated circuits areoften designed to propagate signals in the form of magnetic flux quanta(e.g., via Single Flux Quantum logic) and often employ deliberateinductive couplings to transfer these magnetic signals. These deliberateinductive couplings can exist between adjacent conductive layers in thecircuit and their strength is therefore dependent on the correspondingILD thickness. Circuits that manipulate magnetic signals are alsoparticularly sensitive to unintended inductive couplings between wiringand circuit elements, often referred to as “crosstalks.” The avoidanceand/or minimization of unwanted crosstalks is a crucial aspect ofsuperconducting integrated circuit design. Poor control over ILDthickness can give rise to crosstalks between wiring layers that degradeor completely inhibit circuit performance. For at least these reasons,some implementations of superconducting integrated circuits can greatlybenefit from improved ILD thickness control.

In accordance with the present systems and methods, improved ILDthickness control may be achieved in the fabrication of asuperconducting integrated circuit by employing an additive patterningor Damascene fabrication process. In order to clarify the distinctivefeatures of a superconducting additive patterning process, a typicalstandard subtractive patterning process is first described.

FIG. 9A is a sectional view of a portion of an exemplary superconductingintegrated circuit 900 a during a masking stage of a subtractivepatterning process. Integrated circuit 900 a includes a substrate 930(formed of, for example, silicon, silicon dioxide, sapphire, or asimilar substance), superconducting metal layer 920 (formed of, forexample, niobium), and photoresist mask 910. In the subtractivepatterning process, mask 910 overlies metal layer 920 and effectivelytraces out the desired circuit pattern in metal layer 920. In otherwords, the desired circuit pattern corresponds to regions of metal layer920 that are covered by photoresist mask 910. Those regions of metallayer 920 that are not directly covered by mask 910 will be etched awayand will not form part of the circuit, whereas those regions of metallayer 920 that are directly covered by mask 910 will remain afteretching and become the circuit pattern.

FIG. 9B is a sectional view of a portion of an exemplary superconductingintegrated circuit 900 b. FIG. 9B depicts superconducting integratedcircuit 900 a from FIG. 9A after an etching stage of a subtractivepatterning process. Superconducting integrated circuit 900 b includessubstrate 930 and superconducting metal layer 920, but photoresist mask910 from FIG. 9A has been stripped away. By comparison tosuperconducting integrated circuit 900 a from FIG. 9A, all that remainsof metal layer 920 in superconducting integrated circuit 900 b of FIG.9B are those regions of metal layer 920 that were directly covered byphotoresist mask 910. Metal layer 920 has been subtractively patternedby using, e.g., plasma etching to subtract regions of metal layer 920that were not covered by photoresist mask 910. As discussed in moredetail below, the superconducting metal layer 920 and the photoresistmask 910 are typically etched/removed via different processes.

FIG. 9C is a sectional view of a portion of an exemplary superconductingintegrated circuit 900 c. FIG. 9C depicts superconducting integratedcircuit 900 b from FIG. 9B after a dielectric deposition stage of asubtractive patterning process. Superconducting integrated circuit 900 cincludes substrate 930 and patterned superconducting metal layer 920,but superconducting integrated circuit 900 c also includes dielectriclayer 940 deposited (e.g., by chemical vapor deposition, atomic layerdeposition, or another known technique) over top of patterned metallayer 920 and substrate 930. Dielectric layer 940 serves to protectmetal layer 920 from the external environment and insulate metal layer920 from subsequent metal layers that may be added to integrated circuit900 c. The pattern in metal layer 920 typically produces features andnon-uniformities (e.g., 950) on the surface of dielectric layer 940,which need to be smoothed out before additional layers can be deposited.

FIG. 9D is a sectional view of a portion of an exemplary superconductingintegrated circuit 900 d. FIG. 9D depicts superconducting integratedcircuit 900 c from FIG. 9C after a dielectric planarization stage of asubtractive patterning process. Superconducting integrated circuit 900 dincludes substrate 930, patterned superconducting metal layer 920, anddielectric layer 940. By comparison to superconducting integratedcircuit 900 c from FIG. 9C, dielectric layer 940 of integrated circuit900 d has been planarized to remove any unwanted non-uniformities (e.g.,950 from FIG. 9C) resulting from the underlying pattern in metal layer920. This dielectric planarization may be completed by, for example, aCMP process.

FIG. 9E is a sectional view of a portion of an exemplary superconductingintegrated circuit 900 e. FIG. 9E depicts superconducting integratedcircuit 900 d from FIG. 9D after a second superconducting metal layer960 has been deposited. Superconducting integrated circuit 900 e furtherincludes substrate 930, patterned first superconducting metal layer 920,and dielectric layer 940. Second metal layer 960 is separated from firstmetal layer 920 by dielectric 940, where the distance between secondmetal layer 960 and first metal layer 920 is directly related to thethickness of dielectric layer 940. Dielectric layer 940 is an innerlayer dielectric and the ILD thickness is illustrated in FIG. 9E. Theupper surface of dielectric layer 940 was planarized to remove unwantedfeatures (e.g., 950 from FIG. 9C), thus the ILD thickness of layer 940is determined by this planarization in the subtractive patterningprocess.

The subtractive patterning process illustrated in FIGS. 9A through 9E iscommonly used in both the semiconductor and the superconductorfabrication industries. However, an aspect of this process that isparticularly disadvantageous to the fabrication of superconducting (asopposed to semiconducting) integrated circuits is that the thickness ofeach ILD ends up being defined by a planarization process such as CMP.Planarizing a dielectric layer down to a specific layer thickness is adifficult process to control, at least in part because there is noreference point to indicate when the desired thickness has been reached.In accordance with the present systems and methods, an additivepatterning process may be employed to enhance ILD thickness control inthe fabrication of integrated circuits, and this benefit has particularutility in the fabrication of superconducting integrated circuits. Anadditive patterning process enables enhanced control of ILD thicknessbecause in an additive patterning process, the ILD thickness issubstantially determined by a dielectric deposition operation (via,e.g., chemical vapor deposition, physical vapor deposition, atomic layerdeposition, or a similar process) as opposed to aplanarization/polishing operation. In accordance with the presentsystems and methods, a dielectric deposition process provides better ILDthickness control than a dielectric planarization process.

FIG. 10A is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1000 a during a masking stage of anadditive patterning process in accordance with the present systems andmethods. Integrated circuit 1000 a includes a substrate 1030 (formed of,for example, silicon, silicon dioxide, sapphire, or a similarsubstance), dielectric layer 1040 (formed of, e.g., silicon dioxide),negative photoresist mask 1010, and an etch-stop layer 1070 (which maycomprise, for example, silicon nitride). In the additive patterningprocess, mask 1010 overlies dielectric layer 1040 and effectively tracesout the negative or inverse of the desired circuit pattern in dielectriclayer 1040. In other words, the desired circuit pattern corresponds toregions of dielectric layer 1040 that are not covered by photoresistmask 1010. Those regions of dielectric layer 1040 that are not directlycovered by mask 1010 will be etched away to form open features (e.g.,trenches) while those regions of dielectric layer 1040 that are directlycovered by mask 1010 will remain after etching.

FIG. 10B is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1000 b in accordance with the presentsystems and methods. FIG. 10B depicts superconducting integrated circuit1000 a from FIG. 10A after an etching stage of an additive patterningprocess. Superconducting integrated circuit 1000 b includes substrate1030, dielectric layer 1040, and etch-stop layer 1070, but negativephotoresist mask 1010 from FIG. 10A has been stripped away. Bycomparison to superconducting integrated circuit 1000 a from FIG. 10A,dielectric layer 1040 in superconducting integrated circuit 1000 b ofFIG. 10B has been etched to produce open features (e.g., trenches) 1080that trace out the desired circuit pattern into dielectric layer 1040.Those regions of dielectric layer 1040 that were directly covered byphotoresist mask 1010 remain unetched. The etching into dielectric layer1040 may be controlled by, for example, etch-stop layer 1070, whichprovides an interface between dielectric layer 1040 and substrate 1030through which the etch will not pass. Some embodiments may not includeetch-stop layer 1070 and/or may use substrate 1030 as an etch-stop.

FIG. 10C is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1000 c in accordance with the presentsystems and methods. FIG. 10C depicts superconducting integrated circuit1000 b from FIG. 10B after a metal deposition stage of an additivepatterning process. In addition to the features from FIG. 10B,superconducting integrated circuit 1000 c also includes superconductingmetal layer 1020 deposited (e.g., by electroplating) over top ofpatterned dielectric layer 1040. Superconducting metal layer 1020 maycomprise, for example, niobium or another material capable ofsuperconducting in operation. The deposition of metal layer 1020 fillsthe open features (e.g., 1080 from FIG. 10B) in dielectric layer 1040with superconducting metal. The pattern in dielectric layer 1040 thusserves as a mold for patterning metal layer 1020.

FIG. 10D is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1000 d in accordance with the presentsystems and methods. FIG. 10D depicts superconducting integrated circuit1000 c from FIG. 10C after a metal planarization stage of an additivepatterning process. By comparison to superconducting integrated circuit1000 c from FIG. 10C, metal layer 1020 of integrated circuit 1000 d hasbeen planarized/polished (e.g., by CMP or a similar process) to removeexcess metal down to the top of dielectric layer 1040. After thisplanarization operation, all that remains of metal layer 1020 are thoseportions of metal layer 1020 that fill the open features (e.g., 1080from FIG. 10B) of dielectric layer 1040. Thus, metal layer 1020 has beenpatterned by adding it to the mold in dielectric layer 1040 andpolishing away any excess metal. The planarization/polishing of metallayer 1020 may be configured to stop once the interface between metallayer 1020 and dielectric layer 1040 is reached. A person of skill inthe art will appreciate that planarization/polishing of metal layer 1020may require a specialized slurry that is distinct/modified from theslurry used to planarize/polish a dielectric layer.

FIG. 10E is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1000 e in accordance with the presentsystems and methods. FIG. 10E depicts superconducting integrated circuit1000 d from FIG. 10D after a dielectric layer 1060 has been deposited.Dielectric layer 1060 covers metal layer 1020 and may be deposited byCVD, PVD, ALD, or any other known method of dielectric deposition. Thecombination of dielectric layer 1040 and dielectric layer 1060 togetherforms an inner layer dielectric (“ILD”), whose thickness issubstantially determined by the deposition of dielectric layer 1060. Aspreviously described, greater thickness control may be achieved via adeposition process compared to a planarization process, thus thethickness of the ILD formed by dielectric layers 1040 and 1060 may becontrolled to greater precision than the thickness of the ILD formed bydielectric layer 940 from FIG. 9E. In some embodiments, the uppermostsurface of dielectric layer 1060 may be polished/planarized enough toprovide a smooth surface without having a substantial effect on thethickness of dielectric layer 1060.

FIG. 10F is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1000 f in accordance with the presentsystems and methods. FIG. 10F depicts superconducting integrated circuit1000 e from FIG. 10E after an etch-stop layer 1071 has been deposited.Etch-stop layer 1071 covers dielectric layer 1060 and may be formed of,e.g., silicon nitride deposited by CVD, PVD, ALD, or any other knownmethod of dielectric deposition. Etch-stop layer 1071 ensures that thepatterning (i.e., etching) of additional dielectric layers depositedabove etch-stop layer 1071 will not pass through into dielectric layer1060. Those of skill in the art will appreciate that the definition ofILD thickness may or may not include the thickness of one of moreetch-stop layers.

FIG. 10G is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1000 g in accordance with the presentsystems and methods. FIG. 10G depicts superconducting integrated circuit1000 f from FIG. 10F after a dielectric layer 1043 has been deposited.Dielectric layer 1043 may comprise, for example, silicon dioxidedeposited via CVD, PVD, ALD, or the like. Dielectric layer 1043 isdeposited over etch-stop layer 1071.

FIG. 10H is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1000 h in accordance with the presentsystems and methods. FIG. 10H depicts superconducting integrated circuit1000 g from FIG. 10G after a negative photoresist mask 1011 has beendeposited over dielectric layer 1043. In the additive patterningprocess, mask 1011 overlies dielectric layer 1043 and effectively tracesout the negative or inverse of the desired circuit pattern in dielectriclayer 1043. In other words, the desired circuit pattern corresponds toregions of dielectric layer 1043 that are not covered by negativephotoresist mask 1011. Those regions of dielectric layer 1043 that arenot directly covered by mask 1011 will be etched away to form openfeatures (e.g., trenches) while those regions of dielectric layer 1043that are directly covered by mask 1011 will remain after etching.

FIG. 10I is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1000 i in accordance with the presentsystems and methods. FIG. 10I depicts superconducting integrated circuit1000 h from FIG. 10H after an etching stage of an additive patterningprocess. In superconducting integrated circuit 1000 i, negativephotoresist mask 1011 from FIG. 10H has been stripped away. Bycomparison to superconducting integrated circuit 1000 h from FIG. 10H,dielectric layer 1043 in superconducting integrated circuit 1000 i ofFIG. 10I has been etched to produce open features (e.g., trenches) 1081that trace out the desired circuit pattern into dielectric layer 1043.Those regions of dielectric layer 1043 that were directly covered byphotoresist mask 1011 remain unetched. The etching into dielectric layer1043 is controlled by etch-stop layer 1071, which provides an interfacebetween dielectric layer 1043 and dielectric layer 1042 through whichthe etch will not pass.

FIG. 10J is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1000 j in accordance with the presentsystems and methods. FIG. 10J depicts superconducting integrated circuit1000 i from FIG. 10I after a metal deposition stage of an additivepatterning process. In addition to the features from FIG. 10I,superconducting integrated circuit 1000 j also includes superconductingmetal layer 1021 deposited (e.g., by electroplating) over top ofpatterned dielectric layer 1043. Superconducting metal layer 1021 maycomprise, for example, niobium or another material capable ofsuperconducting in operation. The deposition of metal layer 1021 fillsthe open features (e.g., 1081 from FIG. 10I) in dielectric layer 1043with superconducting metal. The pattern in dielectric layer 1043 thusserves as a mold for patterning metal layer 1021.

FIG. 10K is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1000 k in accordance with the presentsystems and methods. FIG. 10K depicts superconducting integrated circuit1000 j from FIG. 10J after a metal planarization stage of an additivepatterning process. By comparison to superconducting integrated circuit1000 j from FIG. 10J, metal layer 1021 of integrated circuit 1000 k hasbeen planarized/polished (e.g., by CMP or a similar process) to removeexcess metal down to the top of dielectric layer 1043. After thisplanarization operation, all that remains of metal layer 1021 are thoseportions of metal layer 1021 that fill the open features (e.g., 1081from FIG. 10I) of dielectric layer 1043. Thus, metal layer 1021 has beenpatterned by adding it to the mold in dielectric layer 1043 andpolishing away any excess metal. The planarization/polishing of metallayer 1021 may be configured to stop once the interface between metallayer 1021 and dielectric layer 1043 is reached.

As previously described, an additive patterning process may improvecontrol of ILD thickness in a superconducting integrated circuit. Oncethe ILD thickness has been defined, subsequent processing acts mayemploy either an additive patterning or a subtractive patterningapproach as determined by the requirements of the circuit beingfabricated. For example, FIGS. 10G-10K depict a second superconductingmetal layer being patterned by an additive patterning approach, whichmay be advantageous in some circuits (e.g., in circuits that willinclude at least a third metal layer and, consequently, a second ILD,over second metal layer 1021). However, in alternative embodiments(e.g., in circuits where second metal layer 1021 is the topmost metallayer and there is no ILD defined above second metal layer 1021), thedeposition and patterning of second metal layer 1021 may employ asubtractive patterning approach if preferred without adversely affectingILD thickness control in the circuit.

The operations, acts or steps described in FIGS. 10A-10K may be repeatedfor additional dielectric and wiring layers (with additional viaconnections as desired) above layers 1043 and 1021 to provide as manylayers as necessary in any specific integrated circuit design. Theoperations, acts or steps described in FIGS. 10A-10K are summarized inFIG. 11.

FIG. 11 shows a method 1100 for employing an additive patterningtechnique in a superconducting integrated circuit fabrication process torealize improved ILD thickness control in accordance with the presentsystems and methods. Method 1100 includes six operations or acts1101-1106, though those of skill in the art will appreciate that inalternative embodiments certain acts may be omitted and/or additionalacts may be added. Those of skill in the art will appreciate that theillustrated order of the acts is shown for exemplary purposes only andmay change in alternative embodiments. At 1101, a negative photoresistmask is deposited over a first dielectric layer. As described in thecontext of FIG. 10A, the negative photoresist mask traces out theinverse of the desired circuit pattern. The desired circuit patterncorresponds to regions of the first dielectric layer that are notcovered by the photoresist mask. At 1102, the pattern provided by thenegative photoresist mask is etched into the first dielectric layer toproduce open features (e.g., trenches). In some embodiments, the firstdielectric layer may overlie an etch-stop layer through which the etchat 1102 does not pass. At 1103, a superconducting metal layer isdeposited on or over the patterned dielectric layer to fill the openfeatures in the first dielectric layer. The superconducting metal layermay comprise, e.g., niobium and, as described above, the niobium may bedeposited by electroplating, or by standard niobium depositiontechniques depending on the requirements and properties of the specificcircuit being fabricated. In order to fill the open features of thefirst dielectric layer, excess superconducting metal may be depositedsuch that the first dielectric layer is completely coated insuperconducting metal. At 1104, the superconducting metal layer isplanarized (e.g., by a CMP process) down to the level of the firstdielectric layer such that all that remains of the superconducting metallayer are the portions thereof that fill the open features in the firstdielectric layer. At 1105, a second dielectric layer is deposited toproduce a desired ILD thickness. The second dielectric layer isdeposited on top of or over the superconducting metal layer (andconsequently, over the first dielectric layer) to a height that at leastapproximately corresponds to the desired inner layer dielectricthickness. The second dielectric layer may be deposited by, for example,CVD, PVD, ALD, or a similar process. The second dielectric layer may beplanarized to provide a smooth surface, if necessary. At 1106, a secondsuperconducting metal layer is deposited on top of or over the seconddielectric layer, such that the second superconducting metal layer isseparated from the first superconducting metal layer by at least the ILDthickness. In some embodiments, the second superconducting metal layermay be deposited directly on top of or over the second dielectric layer(and, e.g., patterned by a standard subtractive patterning approach:e.g., FIGS. 9A through 9E). In other embodiments, an etch-stop layer mayfirst be deposited directly on top of the second dielectric layer; athird dielectric layer may be deposited on top of or over the etch-stoplayer; the third dielectric layer may be patterned with a negative maskto produce open features; and the second superconducting metal layer maythen be deposited (still on top of or over the second dielectric layer)on top of or over the third dielectric layer to fill the open featuresin the third dielectric layer.

FIGS. 10A through 10K depict a superconducting additive patterningprocess wherein electroplating may be used to deposit superconductingmetal (e.g., niobium) layers and the thickness of the ILD that separatestwo metal layers is substantially controlled by a dielectric depositionprocess (e.g., CVD, PVD, ALD, or the like) as opposed to beingcompletely controlled by a planarization/polishing process. FIGS. 10Athrough 10K, as well as FIG. 11, depict a superconducting Damasceneprocess. However, a person of skill in the art will appreciate that theprocess depicted in FIGS. 10A through 10K (and FIG. 11) is simplified inthat it does not provide any via connections between separate metallayers. In accordance with the present systems and methods, an additiveprocess may also be employed to fabricate a superconducting integratedcircuit having via connections between metal layers. For example, asuperconducting dual Damascene process may be employed to fill both viaholes and open features in dielectric layers with superconducting metal.In accordance with the present systems and methods, a superconductingdual Damascene process may include separate via masking/etching andwiring masking/etching acts. In some embodiments, open vias and wiringfeatures may be filled with superconducting metal simultaneously. Insome embodiments, vias may be masked, etched, and filled in multiplestages.

A superconducting integrated circuit that employs multiplesuperconducting layers often requires superconducting interconnectionsbetween layers. These interconnections are known as “vias.” Hinode etal., Physica C 426-432 (2005) 1533-1540 discusses some of thedifficulties unique to superconducting vias. For example, niobium is asuperconducting metal that is commonly employed as a conductor insuperconducting integrated circuits, but niobium does not naturally fillvia holes very well. This can result in poor contacts between wiringlayers of superconducting integrated circuits that employ niobium. Inparticular, niobium poorly fills holes that have a depth-to-width aspectratio greater than about 0.7:1, or 70%. A person of skill in the artwill appreciate that it is the inherent chemical and/or physical natureof niobium that prevents it from properly filling high aspect ratioholes.

Circuit size is a common design consideration in both semiconductor andsuperconductor integrated circuits, with the goal often being to fitdenser and more sophisticated circuits within limited spatialdimensions. Higher aspect ratio vias are desirable because they canallow for more densely-packed circuits to be developed. According to thestate of the art, the inability to produce high aspect ratiosuperconducting vias currently limits the density and miniaturization ofsuperconducting integrated circuits that employ niobium. In accordancewith the present systems and methods, high aspect ratio (i.e., greaterthan about 0.7:1) niobium vias may be fabricated by employing a“superconducting additive patterning,” “superconducting Damascene,”and/or “superconducting dual Damascene” process.

The various embodiments described herein provide systems and methods fora superconducting dual Damascene process. A superconducting dualDamascene process employs additive patterning and may be similar to thesuperconducting Damascene process depicted in FIGS. 10A through 10K andFIG. 11, with the additional feature that a superconducting dualDamascene process provides superconducting via connections betweenseparate superconducting wiring layers. Via connections may be patternedand formed, for example, within a patterned dielectric before the openfeatures in the dielectric are filled with superconducting metal. Thus,at each metal wiring layer in a multilayered integrated circuit, asuperconducting dual Damascene process may include a wiringmasking/etching operation to produce wiring features, followed by aseparate via masking/etching operation such that vias may be etchedwithin the wiring features.

FIG. 12A is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1200 a during a via masking stage ofa superconducting dual Damascene process in accordance with the presentsystems and methods. Integrated circuit 1200 a includes a Nb-AlOx-Nbtrilayer 1210 carried on or by a substrate 1230 and a dielectric layer1241. Integrated circuit 1200 a is depicted at an intermediate stage ina fabrication process at which trilayer 1210 has already been patterned(via, for example, a photoresist masking and plasma etching technique)to define a Josephson junction 1221 and a wiring component 1222. At thevia masking stage depicted in FIG. 12A, integrated circuit 1200 afurther includes a negative photoresist mask layer 1251 that overliesdielectric layer 1241. Mask 1251 effectively traces out the negative orinverse of the desired locations for via connections to trilayer 1210.In other words, the desired via locations correspond to regions ofdielectric layer 1241 that are not covered by photoresist mask 1251.Those regions of dielectric layer 1241 that are not directly covered bymask 1251 will be etched away to form open holes exposing trilayer 1210while those regions of dielectric layer 1241 that are directly coveredby mask 1251 will remain after etching.

FIG. 12B is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1200 b in accordance with the presentsystems and methods. FIG. 12B depicts superconducting integrated circuit1200 a from FIG. 12A after dielectric etching, metal deposition, andmetal planarization/polishing stages of an additive patterning process,similar to the operations or acts described in FIGS. 10B through 10D.Photoresist mask 1251 and those regions of dielectric layer 1241 thatwere not directly covered by photoresist mask 1251 have been etched away(i.e., in a similar way to that described in going from FIG. 10A to FIG.10B). The resulting holes in dielectric layer 1241 have been filled withsuperconducting metal (i.e., in a similar way to that described in goingfrom FIG. 10B to 10C) to produce first portions of vias 1261 and 1262that provide superconducting connections to Josephson junction 1221 andwiring 1222, respectively. The first portions of superconducting vias1261 and 1262 may, for example, be filled with niobium using a niobiumelectroplating process and may have aspect ratios of approximately0.7:1, less than 0.7:1, or (as enabled by the electroplating fillprocess) greater than 0.7:1. Excess niobium deposited above the uppersurface of dielectric layer 1241 has been removed via a CMP process(i.e., in a similar way to that described in going from FIG. 10C to10D). As will be described in FIGS. 12C through 12G, FIG. 12B shows onlyrespective first portions of superconducting vias 1261 and 1262. Furtherprocessing operations or acts may be employed to add to the structure ofvias 1261 and 1262.

FIG. 12C is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1200 c in accordance with the presentsystems and methods. FIG. 12C depicts circuit 1200 b from FIG. 12Bduring a wiring masking stage of a dual Damascene process after etchstop layers 1271, 1272 and dielectric layers 1242, 1243 have beendeposited. Etch stop layer 1271 is deposited on or over dielectric layer1241 via, for example, a CVD, PVD, or ALD process and may comprise, forexample, SiN. Dielectric layer 1242 is deposited on or over etch-stoplayer 1271 via, for example, a CVD, PVD, or ALD process and maycomprise, for example, SiO₂. The deposition of dielectric layer 1242 maybe used to control the thickness of the resulting ILD that separatestrilayer 1210 from the next metal layer up (not yet shown—see metallayer 1290 in FIG. 12G). In accordance with the present systems andmethods, the additive patterning approach provides improved ILDthickness control because the thickness of a dielectric layer may bemore precisely controlled through a deposition process compared to aplanarization/polishing process. Etch stop layer 1272 is deposited on orover dielectric layer 1242 via, for example, a CVD, PVD, or ALD processand may comprise, for example, SiN. Dielectric layer 1243 is depositedon or over etch-stop layer 1272 via, for example, a CVD, PVD, or ALDprocess and may comprise, for example, SiO₂. At the wiring masking stagedepicted in FIG. 12C, integrated circuit 1200 c further includes anegative photoresist mask layer 1252 that overlies dielectric layer1243. In this additive patterning process, mask 1252 effectively tracesout the negative or inverse of a desired circuit pattern in dielectriclayer 1243. In other words, the desired circuit pattern corresponds toregions of dielectric layer 1243 that are not covered by negativephotoresist mask 1252. Those regions of dielectric layer 1243 that arenot directly covered by mask 1252 will be etched away to form openfeatures (e.g., trenches) while those regions of dielectric layer 1243that are directly covered by mask 1252 will remain after etching. Mask1252 defines a circuit pattern for a metal wiring layer (not shown, seemetal layer 1290 in FIG. 12G) that will lie above trilayer 1210.

FIG. 12D is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1200 d in accordance with the presentsystems and methods. FIG. 12D depicts superconducting integrated circuit1200 c from FIG. 12C after a wiring etching stage of an additivepatterning process. Photoresist mask 1252 from FIG. 12C has beenstripped away and dielectric layer 1243 has been etched to produce openfeatures (e.g., trenches) 1281 and 1282 that trace out the desiredcircuit pattern into dielectric layer 1243. Those regions of dielectriclayer 1243 that were directly covered by photoresist mask 1252 remainunetched. The etching into dielectric layer 1243 is controlled byetch-stop layer 1272, which provides an interface between dielectriclayer 1243 and dielectric layer 1242 through which the etch will notpass, thereby ensuring that the ILD thickness described in FIG. 12Cremains unaffected by this etch.

FIG. 12E is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1200 e in accordance with the presentsystems and methods. FIG. 12E depicts superconducting integrated circuit1200 d from FIG. 12D during a via masking stage of a dual Damasceneprocess. Integrated circuit 1200 e includes negative photoresist mask1253 that overlies patterned dielectric layer 1243 and effectivelytraces out the negative or inverse of the locations for respectivesecond portions of via connections 1261 and 1262 such that vias 1261 and1262 may be extended up to dielectric layer 1243. Etching for the secondportions of vias (e.g., vias 1261 and 1262) may employ an etch chemistry(i.e., a “slurry”) that etches through etch-stop layers 1272 and 1271 aswell as dielectric layer 1242 all at once. Alternatively, etching forthe second portions of vias (e.g., vias 1261 and 1262) may be completedin stages, with a first stage employing an etch chemistry that etchesthrough etch-stop layer 1272, a second stage employing an etch chemistrythat etches through dielectric layer 1242, and a third stage employingan etch chemistry that etches through etch-stop layer 1271. Whether theetching is performed in a single operation or act or in multipleoperations or acts, the etching may be designed to stop when theexisting metal of the respective first portions of vias 1261 and 1262 isexposed. The locations of vias 1261 and 1262 correspond to regions ofetch-stop layer 1272 (above dielectric layer 1242 and etch stop layer1271) within open features 1281 and 1282, respectively, of dielectriclayer 1243 that are not covered by photoresist mask 1253. Those regionsof etch-stop layer 1272 (as well as dielectric layer 1242 and etch-stoplayer 1271) that are not directly covered by mask 1253 will be etchedaway to form open holes exposing the metal of the first portions of vias1261 and 1262, while those regions of etch-stop layer 1272 (as well asdielectric layer 1242 and etch-stop layer 1271) that are directlycovered by mask 1253 will remain after etching. Thus, thesuperconducting dual Damascene process employs additive patterning todefine open features in dielectric layers (in the same way as thesuperconducting Damascene process described in FIGS. 10A through 10K)and further provides masking and etching of via connections within theopen features in the dielectric layers. Vias may be filled in stages,where a second stage provides a superconducting extension to a firststage.

FIG. 12F is a sectional view of a portion of an exemplarysuperconducting integrated circuit 1200 f in accordance with the presentsystems and methods. FIG. 12F depicts superconducting integrated circuit1200 e from FIG. 12E after a via etching stage of a dual Damasceneprocess. Etch-stop layer 1272, dielectric layer 1242, and etch-stoplayer 1271 have all been etched (either all at once or in series asdescribed above) to expose the existing metal in the respective firstportions of vias 1261 and 1262 and provide respective connectionsbetween open features 1281 and 1282 in dielectric layer 1243 and thefirst portions of vias 1261 and 1262. Photoresist mask 1253 from FIG.12E has been stripped away. Those regions of dielectric layer 1243 andetch-stop layer 1272 (as well as dielectric layer 1242 and etch-stoplayer 1271) that were directly covered by photoresist mask 1253 remainunetched. Thus, in the superconducting dual Damascene process, secondportions of vias 1261 and 1262 may be etched into open features 1281 and1282 in a dielectric layer 1243. This enables both the second portionsof vias 1261 and 1262 and the open features 1281 and 1282 in dielectriclayer 1243 to subsequently be filled with superconducting metalsimultaneously.

FIG. 12G is a sectional view of a portion of an exemplary integratedcircuit 1200 g in accordance with the present systems and methods. FIG.12G depicts superconducting integrated circuit 1200 f from FIG. 12Fafter superconducting metal deposition and planarization operations oracts have been completed. Superconducting metal (e.g., niobium) isdeposited on or over dielectric layer 1243 to fill the respectiveportions of superconducting vias 1261 and 1262 and also fill the openfeatures (1281 and 1282 from FIG. 12F) in dielectric layer 1243 toproduce wiring features 1291 and 1292. As previously described, thesuperconducting metal (e.g., niobium) may be deposited by anelectroplating process that enables sufficient fill of the secondportions of via 1261 and 1262 such that the second portions of via 1261and 1262 (and/or vias 1261 and 1262 in their entirety, i.e., thecombined length of the first and second portions of via 1261 and thecombined length of the first and second portions of via 1262) may haveany aspect ratio, including an aspect ratio greater than approximately0.7:1. In order to completely fill open features 1281 and 1282 from FIG.12F, excess metal above the upper surface of dielectric layer 1243 maybe deposited. This excess metal may be removed down to the level ofdielectric layer 1243 via a planarization/polishing process (e.g., CMP,as described in going from FIGS. 10J to 10K). Thus, the metal depositionand planarization operations or acts define a patterned metal wiringlayer 1290 that fills the open features 1281 and 1282 from FIG. 12F indielectric layer 1243 and also fills second portions of vias 1261 and1262 to provide complete via connections between wiring layer 1290 andtrilayer 1210.

The acts described in FIGS. 12A-12G may be repeated for additionaldielectric and wiring layers (with additional via connections asdesired) above layers 1243 and 1290 to provide as many layers asnecessary in any specific integrated circuit design. The operations oracts or steps described in FIGS. 12A-12G are summarized in FIG. 13.

FIG. 13 is a flow-diagram of a method 1300 for implementing asuperconducting dual Damascene process in accordance with the presentsystems and methods. Method 1300 includes ten operations or acts1301-1310, though those of skill in the art will appreciate that inalternative embodiments certain acts may be omitted and/or additionalacts may be added. Those of skill in the art will appreciate that theillustrated order of the acts is shown for exemplary purposes only andmay change in alternative embodiments. At 1301, a negative photoresistmask is deposited on or over a first dielectric layer, where thenegative photoresist mask provides the locations of superconductingvias. The first dielectric layer may overlie a superconducting metallayer, such as a Nb-AlOx-Nb trilayer, as depicted in FIG. 12A. At 1302,the locations of the vias are etched into the first dielectric layer toproduce open holes. The open holes may expose superconducting metal in asuperconducting metal layer beneath the first dielectric layer. At 1303,a superconducting metal layer is deposited on or over the firstdielectric layer to fill the open holes and thereby provide firstportions of the superconducting vias. As previously described, thesuperconducting metal may include niobium and may be deposited, forexample, by an electroplating process. The superconducting metal layeris then planarized (e.g., by CMP) until the upper surface of the firstdielectric layer is exposed. At 1304, a second dielectric layer isdeposited. The deposition of the second dielectric layer may be precededby the deposition of at least one etch-stop layer. In some embodiments,the deposition of the second dielectric layer at 1304 may include, insequence: depositing a first etch-stop layer, depositing an inner layerdielectric, depositing a second etch-stop layer, and depositing thesecond dielectric layer. As previously described, the thickness of theinner layer dielectric is determined by this deposition process and socan be more precisely controlled than in the subtractive patterningprocess, where ILD thickness is controlled by a planarization process.At 1305, a first negative photoresist pattern providing a circuitpattern is deposited on or over the second dielectric layer. At 1306,the circuit pattern is etched into the second dielectric layer toproduce open features (e.g., trenches). When (as described) thedeposition of the second dielectric layer at 1304 is preceded by thedeposition of at least one etch-stop layer, the etch at 1306 may beconfigured to stop at the at least one etch-stop layer. The firstnegative photoresist pattern may then be stripped. At 1307, a secondnegative photoresist mask is deposited on or over the second dielectriclayer providing the locations of superconducting vias within the openfeatures in the second dielectric layer. For example, thesuperconducting vias from operations or acts 1301-1303 may be extendedupwards to connect to the circuit pattern (1305) defined by the openfeatures in the second dielectric layer. Thus, at 1307, the locationswhere the vias from acts 1301-1303 are to connect to the circuit pattern(1305) in the second dielectric layer are defined within the openfeatures in the second dielectric layer. At 1308, the locations of thevias are etched into the second dielectric layer to produce open holeswithin the open features that extend downwards through at least thesecond dielectric layer and expose the tops of the first (filled)portions of the superconducting vias (from act 1303). The secondnegative photoresist mask may then be stripped. At 1309, asuperconducting metal layer is deposited over the second dielectriclayer to fill the open holes providing second portions of the vias andto fill the open features to provide a circuit pattern in the seconddielectric layer. As previously described, the superconducting metal mayinclude niobium and may be deposited by an electroplating process thatfacilitates filling the open holes and enables vias having any aspectratio (e.g., an aspect ratio greater than about 0.7:1) to be produced.Act 1309 simultaneously forms wiring elements in the second dielectriclayer and completes via connections from the wiring elements in thesecond dielectric layer (to, e.g., a wiring layer that lies beneath thefirst dielectric layer). At 1310, the superconducting metal layer isplanarized or polished (e.g., by a CMP process) to remove any excessmetal and expose the upper surface of the second dielectric layer. Ifnecessary to produce additional layers in the integrated circuit, athird dielectric layer may be deposited and acts 1301-1310 may then berepeated on top of or over the third dielectric layer.

The superconducting Damascene and/or dual Damascene processes depictedin FIGS. 10A through to 10K, FIG. 11, FIGS. 12A through 12G, and FIG. 13may be used to fabricate complete superconducting integrated circuitswithout ever needing to etch the superconducting metal itself. Such maybe advantageous for circuits that employ niobium as the superconductingmetal because techniques for etching niobium are less developed in theart than techniques for etching other materials, such as aluminum (dueto the extensive use of aluminum in the semiconductor industry).Furthermore, etching of niobium can result in the formation of niobiumoxides and/or other compounds that can adversely affect circuitoperation and performance. Thus, the superconducting Damascene and/ordual Damascene approaches described herein have the additional benefitof reducing (and in some cases, eliminating) the formation of unwantedniobium oxides.

The superconducting Damascene and dual Damascene processes describedabove implement certain process changes compared to standard photoresistmasking and plasma etching (i.e., standard subtractive patterning)which, when taken all together, can provide specific benefits tosuperconducting integrated circuit fabrication. However, in accordancewith the present systems and methods, some of the processes describedabove may be individually incorporated into an otherwise standardphotoresist masking and plasma etching process to realize certainbenefits. For example, improved via fill and via critical currentcontrol may be achieved by employing electroplating to deposit niobiumeven in a subtractive patterning process. Such a process may involvelittle change from the process outlined in FIGS. 9A through 9E exceptthat at the niobium deposition stage(s) an electroplating process isemployed as opposed to a more typical CVD, PVD, or ALD process. Asdescribed above, depositing niobium via an electroplating process maybetter fill via connections (compared to depositing niobium via a CVD,PVD, or ALD-type process) and allow vias having high aspect ratios(e.g., greater than about 0.7:1) to be reliably fabricated.

Another example of an individual aspect of the superconducting Damasceneand/or dual Damascene process described above that may be integratedinto an otherwise standard subtractive patterning process is: definingILD thickness by dielectric deposition as opposed to by dielectricplanarization. In accordance with the present systems and methods, anotherwise typical subtractive patterning process (e.g., as shown inFIGS. 9A to 9E) may be modified such that ILD thickness is determined bya dielectric deposition process as opposed to by a dielectricplanarization process. Such a modification may include, for example,extending the dielectric planarization portrayed in going from FIG. 9Cto FIG. 9D by planarizing dielectric layer 940 all the way down to thetop of metal layer 920. An additional dielectric deposition may then beemployed to raise the level of dielectric layer 940 until the desiredILD thickness is reached (e.g., to the level depicted in FIG. 9E). Asdescribed previously, dielectric deposition may provide better thicknesscontrol than dielectric planarization. In some embodiments, planarizingdielectric layer 940 all the way down to the top of metal layer 920 mayinvolve CMP and may employ a highly selective slurry (“HSS”) as opposedto a standard slurry.

Throughout this specification, reference is often made to a substrateformed of, for example, silicon, silicon dioxide, sapphire, or a similarmaterial (such as quartz). In the semiconductor industry, doped siliconis often employed as a substrate or carrier in an integrated circuitbecause doping can facilitate the fabrication process(es). However, in asuperconducting integrated circuit, such dopants may be a source ofunwanted noise and/or they may raise the heat capacity of the siliconsubstrate, which is particularly undesirable in superconducting circuitswhere an important function of the substrate is to help cool theconducting metals into the superconducting regime. Thus, it may bepreferable to employ pure, undoped silicon as a substrate or carrier ina superconducting integrated circuit.

While many different materials may be employed as a substrate in asuperconducting integrated circuit, silicon with a top layer of silicondioxide is commonly used. A top layer of silicon dioxide is often added,at least in part, because silicon on its own is transparent, which canmake it difficult to run standard lithographic processes. However, insuperconducting circuits that are particularly sensitive to noise (e.g.,in superconducting qubit circuits such as superconducting quantumprocessors), this silicon-to-silicon-dioxide interface can become anundesirable source of noise. In accordance with the present systems andmethods, it can be advantageous in some applications to fabricate asuperconducting integrated circuit on a substrate comprising siliconwith a top layer of an alternative material, such as aluminum oxide,instead of silicon with a top layer of silicon dioxide.

FIG. 14 is a sectional view of a portion of a substrate 1400 for use ina superconducting integrated circuit in accordance with the presentsystems and methods. Substrate 1400 comprises two layers: a base layer1410 of silicon (e.g., standard doped silicon or pure, undoped siliconas described above) and a top layer 1420 of aluminum oxide. In someembodiments, base layer 1410 may comprise sapphire, quartz, or anyalternative material suitable as a substrate. Aluminum oxide layer 1420is non-transparent (e.g., translucent, semi-transparent, or opaque) andtherefore facilitates the use of substrate 1400 in conjunction withstandard lithographic processing techniques. Furthermore, thesilicon-to-aluminum-oxide interface may provide improved noisecharacteristics (i.e., act as a lesser source of noise) compared to thesilicon-silicon-dioxide interface commonly employed in substrates. Anadditional benefit of aluminum oxide top layer 1420 is that aluminumoxide may serve as a better etch-stop compared to silicon dioxide andtherefore enable more precise etching and patterning of material (e.g.,superconducting metal, such as niobium) deposited over aluminum oxidetop layer 1420.

As described previously, the quality of a trilayer Josephson junctionmay be degraded (specifically, the quality of the insulating barrier,e.g., AlOx, may be degraded) if the junction is heated aboveapproximately 200° C. This means that once a Josephson junction trilayerhas been deposited in a superconducting integrated circuit, it can beadvantageous to perform all subsequent processing operations or acts atlower-than-standard temperatures (i.e., <200° C.) to preserve Josephsonjunction quality. In the semiconductor industry, dielectrics aretypically deposited at high temperature (e.g., above 400° C.) to improvepurity and smoothness. However, in superconducting integrated circuitsemploying Josephson junctions, depositing a dielectric layer over aJosephson junction at such a high temperature may adversely affect theJosephson junction itself. Accordingly, it can be advantageous to employlower-temperature dielectric deposition processes in circuits thatinclude Josephson junction trilayers. An example of a lower temperaturedielectric process is a low temperature tetraethyl orthosilicate(“TEOS”) dielectric deposition process. TEOS is often used in the art asa precursor to silicon dioxide, but at temperatures (e.g., 650-850° C.)that may adversely affect Josephson junction quality. In accordance withthe present systems and methods, a TEOS dielectric deposition process(e.g., a CVD TEOS process or a plasma-enhanced CVD TEOS process) may beperformed at significantly lower temperature (e.g., around 200° C.) whenapplied over a trilayer Josephson junction in order to preserveJosephson junction quality.

Many of the embodiments described herein are directed towardsapplications in superconducting quantum computation. Those of skill inthe art will appreciate that the requirements (e.g., tolerable levels ofnoise) for manipulating quantum information may be more stringent thanthe requirements for manipulating non-quantum information. Thus, whilethe various embodiments described herein are particularly well-suitedfor use in the fabrication of a superconducting quantum processor, theseteachings may be applied to any application incorporatingsuperconducting integrated circuitry (including applications for whichthe performance criteria are less stringent). For example, the variousteachings provided herein may be applied in single-flux quantum (SFQ)circuits or any circuit employing a Josephson junction. In someinstances, applying the present systems and methods in non-quantumcomputing applications may allow certain constraints to be relaxed. Anapplication of SFQ is likely to be less sensitive to noise than aquantum computing application, and as such a lower temperaturedielectric process may readily be applied to an SFQ circuit in order topreserve Josephson junction quality with less regard for the resultantincrease in dielectric defects.

In addition to lower-temperature dielectric deposition processes, metaldeposition processes that occur after (e.g., on top of or above) aJosephson junction trilayer can also cause the junction to heat to thepoint of insulating barrier degradation. In accordance with the presentsystems and methods, unwanted heating of a Josephson junction trilayerby a subsequent metal deposition process may be avoided by performing ametal deposition process in multiple stages and allowing the system tocool in between stages.

FIG. 15 shows a method 1500 for performing multi-staged metal depositionin accordance with the present systems and methods. Method 1500 includesfive operations or acts 1501-1505, though those of skill in the art willappreciate that in alternative embodiments certain acts may be omittedand/or additional acts may be added. Those of skill in the art willappreciate that the illustrated order of the acts is shown for exemplarypurposes only and may change in alternative embodiments. At 1501, afirst stage of the metal deposition process is initiated to deposit afirst portion of a metal layer on an integrated circuit. The metal beingdeposited may be a superconducting metal such as niobium or aluminum andthe metal deposition process may employ any deposition technique, suchas CVD, PVD, or ALD. The process of depositing the metal may cause theintegrated circuit to heat. At 1502, the first stage of the metaldeposition process is stopped to prevent the integrated circuit fromheating to the point where any existing components that are sensitive tohigh temperatures (e.g., Josephson junction trilayers) may be damaged.The temperature of the integrated circuit (or the temperature of thechamber in which the deposition takes place) may be monitored during thefirst stage of the metal deposition and the metal deposition process maybe stopped when the temperature being monitored is seen to approach orexceed a predetermined threshold. Alternatively, a target depositiontime may be determined in advance based on previous data and/orcalculations and the first stage of the metal deposition process may bestopped once the pre-defined target deposition time has elapsed. At1503, the integrated circuit is cooled. Since the metal depositionprocess has been stopped, the circuit may cool passively with thepassage of time and act 1503 may simply involve waiting until thecircuit has cooled. In some embodiments, the circuit may be cooledactively by filling the deposition chamber with an inert gas, such asargon. In general, the higher the pressure of the gas the betterthermalization it will provide, but of course the pressure is limited byfactors such as the strength of the chamber, the cooling time, etc. At1504, an additional stage of the metal deposition process is initiatedto deposit an additional portion of the metal layer (i.e., to resumedepositing the metal layer) on top of or over the previous portion ofthe metal layer. The additional stage of the metal deposition maycontinue until either the total desired metal layer thickness (i.e., thesum of the thickness of the first portion of the metal layer plus thethickness of the additional portion(s) of the metal layer) is depositedor until the threshold temperature/time is reached again. At 1505 a, thetotal desired metal layer thickness is achieved and the multi-stagemetal layer deposition process is complete. At 1505 b, the temperatureof the circuit reaches the threshold temperature before the totaldesired metal thickness is achieved so acts 1502-1505 are repeated untilthe total desired metal layer thickness is achieved.

The various embodiments described herein provide systems and methods forthe fabrication of multilayered superconducting integrated circuits.Such circuits are typically fabricated layer by layer (e.g., one layerat a time, and with via connections between layers), thus it isimportant to ensure that the features in each layer are properly alignedwith the features of the layer or layers above and/or below. Forexample, a feature in a second layer that is to be connected by a viaconnection to a feature in a first layer (where the second layer isabove the first layer) typically needs to be properly aligned above thefeature in the first layer. Throughout the semiconducting fabricationindustry, a process known as “open frame mask and etch” is oftenemployed to provide this alignment. The open frame mask and etchtechnique involves marking the substrate with a “zero mark” or“alignment mark” before any conducting or insulating layers aredeposited therein. The zero mark then needs to be “seen” before eachsubsequent layer is deposited over the substrate. This means that aftera conducting layer has been deposited, the region of the conductinglayer that overlies the zero mark in the substrate needs to be etchedaway to expose the zero mark so that the patterning of the conductinglayer can be properly aligned. Thin dielectric layers (e.g., SiO₂) aretypically sufficiently transparent to enable the zero mark to be seenwithout etching.

Thus, in a superconducting version of the open frame mask and etchalignment technique, a superconducting metal layer is deposited and afirst photoresist mask layer is deposited on or over the superconductingmetal layer that completely covers the surface of the superconductingmetal layer except for an open region in the vicinity of the zero mark.An etch is then applied that etches away the exposed region in thesuperconducting metal layer to reveal the zero mark in the substrate.Any remaining photoresist is then stripped away, and a secondphotoresist mask layer is then deposited on or over the superconductingmetal layer, where the second photoresist mask layer provides thecircuit pattern for the superconducting metal layer, aligned to theexposed zero mark in the substrate. This process is repeated for eachsubsequent superconducting metal layer in the integrated circuit stack.As stated above, the open frame mask and etch process is commonly usedin the semiconducting industry; unfortunately, when the same techniqueis applied using superconducting metal such as niobium, it has beenfound that exposing the same superconducting (e.g., niobium) metal layerto two photoresist masking and etching operations (i.e., a first forexposing the zero mark and then a second for applying the circuitpattern aligned to the zero mark) can result in the formation ofundesirable residues (e.g., photoresist residues and/or metallicresidues, Nb defects, etc.) on the surface of the superconducting metallayer which can adversely affect the performance of the integratedcircuit. Thus, there is a need in the art for an alternative method ofaligning the multiple layers in a superconducting integrated circuitthat reduces the number of masking and etching operations persuperconducting metal layer.

In accordance with the present systems and methods, the layers of amultilayered superconducting integrated circuit may be properly alignedduring fabrication by etching a respective alignment mark into eachdielectric layer. In this approach, no zero markings are required on thesubstrate. Instead, a first superconducting metal layer may be patternedto include circuit wiring and an alignment mark. A dielectric layer maybe deposited on or over the first superconducting metal layer. Thedielectric layer, for example SiO₂, may be sufficiently transparent toenable the alignment mark in the first superconducting metal layer to bediscerned through the dielectric layer. The dielectric layer may then bepatterned and etched to provide holes that expose specific portions ofthe first superconducting metal layer, where these holes will ultimatelycorrespond to superconducting via connections to the firstsuperconducting metal layer. Such patterning and etching of thedielectric layer is standard. However, in accordance with the presentsystems and methods, the pattern in the dielectric layer may alsoinclude an alignment mark that overlies the alignment mark in the firstsuperconducting metal layer. This alignment mark is etched into thedielectric layer and can easily be made to overlie the alignment mark inthe first superconducting metal layer because the dielectric layer issufficiently transparent. A second superconducting metal layer is thendeposited on top of or over the dielectric layer. The deposition of thesecond superconducting metal layer fills (or at least, partially fills)the patterned holes in the dielectric layer to provide superconductingvia connections to the first superconducting metal layer. The depositionof the second superconducting metal layer also at least partially fillsthe alignment mark etched into the dielectric layer. The alignment markmay be designed (e.g., in size and/or in shape) so that the fact that itis filled by the deposition of the second superconducting metal layercauses the alignment mark to be discernible in the upper surface of thesecond superconducting metal layer. For example, if the alignment markis sufficiently large, an impression of the alignment mark may bediscernible in the upper surface of the second superconducting metallayer as the metal “sinks in” to fill the mark. Thus, an alignment markin the first superconducting metal layer is recreated in the overlyingdielectric layer such that it leaves an impression in the secondsuperconducting metal layer. The photoresist mask providing the patternfor the second superconducting metal layer may then be deposited andaligned to the impression of the alignment mark. This process may thenbe repeated for any number of additional layers.

FIG. 16 is a sectional view of a portion of an exemplary superconductingintegrated circuit 1600 showing an impression 1680 of an alignment markin a superconducting metal layer 1622. Circuit 1600 includes substrate1630, upon which is deposited a first superconducting metal layer 1621.Layer 1621 has been patterned (by, for example, masking and etching) todefine wiring features 1631, 1632 and an alignment mark 1650. Dielectriclayer 1640 has been deposited over patterned metal layer 1621.Dielectric layer 1640 has also been patterned (e.g., by masking andetching) to expose alignment mark 1650 and wiring features 1631, 1632 ofmetal layer 1621. Metal layer 1622 has been deposited over dielectriclayer 1640 and filled the open features in dielectric layer 1640 toproduce superconducting vias 1661, 1662 and an impression 1680 in theupper surface of metal layer 1622 that overlies alignment mark 1650.Impression 1680 may arise due to the size and/or shape of alignment mark1650. In various embodiments and depending on the nature of alignmentmark 1650, impression 1680 may include at least one recess, at least oneprotrusion, multiple recesses, multiple protrusions, and/or acombination of at least one recess and at least one protrusion.Impression 1680 may be discernible during subsequent lithographicprocessing of metal layer 1622 and thereby serves as a point ofreference for aligning the deposition of a photoresist mask layer on topof metal layer 1622. If an additional metal layer (not shown) is to bedeposited on top of or over metal layer 1622 (e.g., after deposition ofan additional dielectric layer) then impression 1680 may be patternedinto a new alignment mark in metal layer 1622 (or, alternatively,impression 1680 may be etched away and a new alignment mark may bepatterned elsewhere into metal layer 1622). This process of patterningalignment marks into dielectric layers to leave impressions in overlyingmetal layers allows multiple layers to be aligned in a stack without theadditional metal-patterning operations or acts associated with openframe mask and etch alignment techniques. In this way, potentiallyundesirable photoresist and/or metallic residues may be avoided orreduced. In some embodiments, the alignment mark(s) 1650 may be designedto provide a distinctive impression in 1680 that may be easilydistinguished from other surface features inherent to the fabricationprocess. For example, the alignment mark(s) may be significantly (e.g.,multiple times) larger than features in the circuit pattern itselfand/or may embody a distinct shape or shapes. The process described inthe context of FIG. 16 is summarized in FIG. 17.

FIG. 17 shows a method 1700 for aligning multiple layers in amulti-layered superconducting integrated circuit without using an openframe and match technique in accordance with the present systems andmethods. Method 1700 includes five operations or acts 1701-1705, thoughthose of skill in the art will appreciate that in alternativeembodiments certain acts may be omitted and/or additional acts may beadded. Those of skill in the art will appreciate that the illustratedorder of the acts is shown for exemplary purposes only and may change inalternative embodiments. At 1701, a first superconducting metal layer ispatterned to include at least one alignment mark. As previouslydescribed, the alignment mark may be large or otherwise specificallydesigned to leave a recognizable impression in an overlying metal layersuch that the impression may be discerned during a subsequentlithography stage. At 1702, a first dielectric layer is deposited on orover the first superconducting metal layer. At 1703, the firstdielectric layer is patterned (e.g., masked and etched) to define openholes at the locations of vias and to expose the at least one alignmentmark. The first dielectric layer may be at least partially transparentand the at least one alignment mark may be discernible through the firstdielectric layer. At 1704, a second superconducting metal layer isdeposited on or over the first dielectric layer to at least partiallyfill open holes and provide via connections. The second superconductingmetal layer also covers the at least one alignment mark, which leaves acorresponding impression on the opposite (i.e., the exposed) surface ofthe second superconducting metal layer. At 1705, a photoresist mask isaligned to the impression of the alignment mark on the secondsuperconducting metal layer (e.g., on the exposed surface of the secondsuperconducting metal layer). The photoresist mask may then be depositedon or over the second superconducting metal layer, and acts 1701-1705may be repeated if additional superconducting metal layers are required.The alignments marks in subsequent layers may overlie the alignment markin the first superconducting metal layer, or the alignment marks insubsequent metal layers may be positioned in locations that do notoverlie the alignment mark in the first superconducting metal layer.

The issues of alignment described above may, in some instances, beavoided if an additive patterning process (such as a Damascene processor a dual Damascene process) is employed.

US Patent Publication 2011-0089405 describes the use of platinum as aresistor material in superconducting integrated circuits. In accordancewith the present systems and methods, platinum may be deposited via asputter process and a thin layer of an intermediate material, such astitanium, may be used to improve adhesion between the platinum and thesurface upon which the platinum is being deposited. That is, if theplatinum is to be used as a resistor deposited on or over a dielectricmaterial such as SiO₂, a thin “adhesion layer” (formed of, e.g.,titanium) may first be deposited (via, e.g., a sputter process) on thedielectric surface and the platinum may then be deposited directly onthe adhesion layer. The titanium-platinum (TiPt) stack may then bepatterned and etched via a lithographic process using, for example, Cl₂and SF₆ chemistry in the etchant.

As previously described, some superconducting metals, including niobium,do not naturally fill via holes very well. This can result in poorelectrical contacts between wiring layers of superconducting integratedcircuits that employ vias. In accordance with the present systems andmethods, via-fill may be improved by changing the etch profile of thevia hole. Typically, a via hole is etched to form substantially smooth,substantially vertical sidewalls (see, e.g., vias 1261 and 1262 in FIGS.12E to 12G and vias 1661 and 1662 in FIG. 16). In accordance with thepresent systems and methods, forming via holes having textured and/ornon-vertical sidewalls may improve via fill when superconducting metal,such as niobium, is subsequently deposited. Via holes having texturedand/or non-vertical sidewalls may therefore improve electricalconnections between wiring layers in multilayered superconductingintegrated circuits and/or enable higher aspect ratio vias (e.g., aspectratios greater than about 0.7:1) to be fabricated. An example of a viahaving non-vertical sidewalls is a tapered via. Techniques forfabricating tapered vias are generally known in the semiconductorindustry and, in accordance with the present systems and methods, manyof the same techniques (e.g., etchant chemistry, etc.) may be employedin the fabrication of superconducting vias to improve superconductingelectrical connections between layers in a superconducting integratedcircuit.

FIG. 18 is a sectional view of a portion of an exemplary superconductingintegrated circuit 1800 showing superconducting via 1860 havingnon-vertical sidewalls 1861 and 1862, in accordance with the presentsystems and methods. As illustrated in FIG. 18, sidewalls 1861 and 1862are tapered such that via 1860 is wider at the top and narrower at thebottom. Tapered superconducting via 1860 provides a superconductingelectrical connection between superconducting wiring layer 1852 andsuperconducting wiring layer 1851. As illustrated, superconductingwiring layer 1851 provides the top/counter electrode of a Josephsonjunction. While tapered superconducting via 1860 may be etched accordingto known techniques for etching tapered vias in the semiconductorindustry, in accordance with the present systems and methods, thesuperconducting metal of superconducting wiring layer 1851 (e.g.,niobium) may be used as an etch stop in the etching of tapered via 1860(as opposed to materials more commonly used as an etch stop in thesemiconductor industry). Once the tapered profile of via 1860 is etchedinto dielectric layer 1840, superconducting metal (e.g., niobium) 1852may be deposited over dielectric layer 1840 to fill via 1860. Thetapered profile of sidewalls 1861 and 1862 may facilitate improvedfilling of via 1860 with superconducting metal 1852 compared to, forexample, a via profile having vertical sidewalls. An improvedsuperconducting electrical connection between superconducting metallayer 1852 and superconducting metal layer 1851 may thus be established,where superconducting metal layer 1851 (e.g., niobium) serves as both anetch-stop during etching of tapered via 1860 and as a superconductingwiring layer in circuit 1800.

In some cases, poor fill of superconducting vias may be a result ofover-etching a via hole and digging into the underlying superconductingmetal layer. Such over-etching can produce a trench in the metal (e.g.,niobium) beneath the sidewalls of the via and, once the via issubsequently filled with superconducting metal (e.g., niobium), resultin thin physical connections between the superconducting metal on thevia sidewalls and the underlying superconducting metal beneath the via.Thin physical connections typically result in poor electricalconnections. In accordance with the present systems and methods,over-etching into underlying superconducting metal may be reduced bydepositing a protective cap layer over the underlying superconductingmetal. For example, a superconducting metal layer may be capped with athin layer of protective material (such as titanium nitride or siliconnitride). When a via is subsequently etched over top of the cappedsuperconducting metal layer, the protective cap layer may preventover-etching into the superconducting metal layer and ultimately providebetter electrical connections between superconducting metal on the viasidewalls and the superconducting metal layer beneath. The protectivecap layer may be superconducting material. Titanium nitride isparticularly well-suited to provide a capping layer on niobium wiringlayers because: titanium nitride can superconduct below about 4.2K,titanium nitride oxidizes less than niobium, and titanium nitride etcheswell with niobium etch chemistry but serves as a good etch stop for SiO₂etch chemistry (i.e., during via etch). In some applications, it isadvantageous to ensure that a protective capping layer formed ofsuperconducting material does not create superconductive shorts betweenelements of a patterned superconducting metal layer (e.g., betweensuperconductive paths or traces in a superconducting wiring layer). Suchshorts may be avoided by, for example, depositing the protective cappinglayer over a superconducting metal layer prior to patterning thesuperconducting metal layer such that the capping layer is alsopatterned during the patterning process.

FIG. 19 shows a method 1900 for forming a superconducting via inaccordance with the present systems and methods. Method 1900 includessix operations or acts 1901-1906, though those of skill in the art willappreciate that in alternative embodiments certain acts may be omittedand/or additional acts may be added. Those of skill in the art willappreciate that the illustrated order of the acts is shown for exemplarypurposes only and may change in alternative embodiments. At 1901, afirst superconducting metal layer is deposited. The firstsuperconducting metal layer may comprise a superconducting metal, suchas niobium or aluminum, and may be deposited over a substrate ordielectric layer, or over an insulating barrier such as an aluminumoxide layer. The first superconducting metal layer may, for example, bea superconducting wiring layer or a superconducting counter electrodelayer in a Josephson junction. At 1902, a superconducting protectivecapping layer is deposited over the first superconducting metal layer.The superconducting protective capping layer may comprise, for example,titanium nitride, niobium titanium nitride, or any other suitablematerial. At 1903, the first superconducting metal layer is patterned(via, for example, a photoresist mask and etching process as describedpreviously). Patterning the first superconducting metal layernecessarily includes patterning the superconducting protective cappinglayer with the same pattern such that the portions of the firstsuperconducting metal layer that remain after the patterning process(e.g., a wiring pattern in the superconducting metal layer, and/or thedefinitions of Josephson junction counter electrodes in thesuperconducting metal layer) retain the superconducting protectivecapping layer thereover. At 1904, a dielectric layer is deposited overthe patterned superconducting protective capping layer and firstsuperconducting metal layer. The dielectric layer may comprise, forexample, silicon dioxide or a hybrid dielectric layer as describedpreviously. At 1905, a hole is etched through the dielectric layer toexpose a portion of at least one of the superconducting protectivecapping layer and the first superconducting metal layer. The etchingprocess may stop when the superconducting protective capping layer isexposed, or the etching process may continue at least partially into thesuperconducting protective capping layer, or the etching process maystop when the first superconducting metal layer is exposed. In any case,the superconducting protective capping layer may help to maintain theshape of the hole during the etching process and prevent over-etchinginto the first superconducting metal layer. At 1906, a secondsuperconducting metal layer is deposited over the dielectric layer. Thesecond superconducting metal layer may at least partially fill the holethrough the dielectric layer and form a superconducting electricalconnection (i.e., a superconducting via) with at least one of thesuperconducting protective capping layer and the first superconductingmetal layer.

FIG. 20 is a sectional view of a portion of a superconducting integratedcircuit 2000 including a superconducting protective capping layer 2051over a superconducting metal layer 2021 in accordance with the presentsystems and methods. Superconducting integrated circuit 2000 furtherincludes superconducting metal layer 2022 that is separated fromsuperconducting metal layer 2021 by dielectric layer 2031.Superconducting metal layers 2021 and 2022 may each include, forexample, niobium. Superconducting metal layer 2021 is superconductivelycoupled to superconducting metal layer 2022 through superconducting via2061. In FIG. 19, via 2061 is shown partially etched intosuperconducting protective capping layer 2051 without exposingsuperconducting metal layer 2021. As described above, superconductingprotective capping layer 2051 may prevent over-etching intosuperconducting metal layer 2021 during the formation of superconductingvia 2061 and thereby improve the superconducting electrical connectionbetween superconducting metal layers 2021 and 2022. Superconductingprotective capping layer 2051 may include, for example, titaniumnitride, niobium titanium nitride, or any suitable material as describedabove.

As described previously, the behavior of a Josephson junction isinfluenced by a property called its critical current. The criticalcurrent of a Josephson junction is the maximum amount of current (for agiven external magnetic field, typically reported at zero externalmagnetic field) that can flow through the junction without causing thejunction to switch into the voltage state. The critical current of aJosephson junction is dependent on a number of factors, including thearea of the junction and the thickness of the insulating barrier. For agiven thickness of insulating barrier, the larger the area of thejunction the larger its critical current. Likewise, for a given area ofjunction, the larger the thickness of the insulating barrier the lowerits critical current. In superconducting integrated circuits that employtrilayer Josephson junctions, a single trilayer having a uniform barrierthickness is typically deposited and junctions of different criticalcurrents are realized by patterning the trilayer to form junctions ofdifferent areas. For example, if a circuit requires a first Josephsonjunction having a first critical current and a second Josephson junctionhaving a second critical current where the second critical current islarger than the first critical current, then the second Josephsonjunction may be designed and laid out to have a larger area than thefirst Josephson junction. This approach is suitable for relatively smallcircuits and/or for circuits that employ Josephson junctions havingsimilar critical currents, but can be problematic for large complicatedcircuits and/or circuits that employ Josephson junctions spanning a widerange of critical currents. For example, in a circuit that makes use ofa single trilayer to form a first set of Josephson junctions having afirst critical current and a second set of Josephson junctions having asecond critical current, where the second critical current is muchlarger than the first critical current, the area of each junction in thesecond set of junctions needs to be much larger than the area of eachjunction in the first set of junctions. The large areas of the junctionsin the second set of Josephson junctions may undesirably increase thetotal footprint (i.e., area) of the integrated circuit itself, which canintroduce complications in connecting the circuit to an electricalinput/output system and/or in shielding the circuit from ambientmagnetic fields, and can ultimately render the integrated circuit toolarge for its intended application. In some applications, it may bepossible to overcome these problems by depositing a separate trilayerwithin the integrated circuit, where the second trilayer employs aninsulating barrier thickness that is different from that of the firsttrilayer. However, depositing a second trilayer significantly increasesthe number of layers in the integrated circuit stack and, accordingly,the number of processing steps required in the fabrication of the stack.Such can increase the likelihood of defects and generally reduce thelikelihood of yielding a fully functional circuit. It can also be verychallenging to achieve a uniform insulating barrier thickness in asecond trilayer because the second trilayer must necessarily bedeposited at a higher layer in the circuit stack (i.e., the first andsecond trilayers cannot both be deposited on the substrate) where thesurface upon which the second trilayer is deposited may be less smoothand less level than the substrate.

The thickness of the insulating barrier in a Josephson junctioninfluences a parameter known as the “critical current density,” or“J_(c),” of the Josephson junction. J_(c) is essentially a measure ofthe critical current per area of the Josephson junction, where a thickerinsulating barrier typically produces a lower J_(c) and a thinnerinsulating barrier typically produces a higher J_(c).

In accordance with the present systems and methods, Josephson junctionshaving different critical currents may be realized in a singlesuperconducting integrated circuit by replacing the Josephson junctiontrilayer with a Josephson junction “pentalayer” having two insulatingbarriers of different thicknesses. A Josephson junction “pentalayer” maycomprise five layers: a first layer of superconducting material (e.g.,niobium) serving as a first base electrode, a first insulating barrier(e.g., aluminum oxide, including aluminum oxide grown on aluminum asdescribed previously) having a first J_(c), a second layer ofsuperconducting material (e.g., niobium) serving as both a first counterelectrode and a second base electrode, a second insulating barrier(e.g., aluminum oxide, including aluminum oxide grown on aluminum)having a second J_(c) that is different from the first J_(c), and athird layer of superconducting material (e.g., niobium) serving as asecond counter electrode. As will be described in more detail, ingeneral it may be advantageous for the second insulating barrier to bethicker than the first insulating barrier such that the second J_(c) isless than the first J_(c).

FIG. 21A is a sectional view of a portion of a superconductingintegrated circuit 2100 a including a Josephson junction pentalayer 2110in accordance with the present systems and methods. Pentalayer 2110comprises: first base electrode 2111 formed of niobium, first insulatingbarrier 2112 formed of aluminum oxide (as described previously, a layerof aluminum may be positioned in between first base electrode 2111 andfirst insulating barrier 2112 for the purpose of growing firstinsulating barrier 2112), first counter electrode 2113 formed ofniobium, where first counter electrode 2113 may also serve as a secondbase electrode, second insulating barrier 2114 formed of aluminum oxide(as described previously, a layer of aluminum may be positioned inbetween first counter electrode 2113 and second insulating barrier 2114for the purpose of growing second insulating barrier 2114), and secondcounter electrode 2115 formed of niobium. As illustrated in FIG. 21A,first insulating barrier 2112 is substantially thinner than secondinsulating barrier 2114. As a result, first insulating barrier 2112 hasa substantially higher J_(c) than second insulating barrier 2114. Inaccordance with the present systems and methods, pentalayer 2110 may bepatterned to form Josephson junctions whose critical currents aredetermined by either insulating barrier 2112 or insulating barrier 2114.Therefore, pentalayer 2110 enables multiple J_(c)s to be used indefining Josephson junction circuit elements while employing fewerlayers and fewer processing steps compared to a complete secondtrilayer. Furthermore, pentalayer 2110 minimizes the number of layersbetween second insulating barrier 2114 and the substrate so that secondinsulating barrier 2114 may be more level and more uniform in thicknessthan if it was deposited higher in the circuit stack.

As described above, first insulating barrier 2112 has a substantiallyhigher J_(c) (i.e., is thinner) than second insulating barrier 2114. Inaccordance with the present systems and methods, for most applicationsit is advantageous for the higher-J_(c) barrier in a Josephson junctionpentalayer to be positioned below the lower-J_(c) barrier (oralternatively, for the lower-J_(c) barrier to be positioned above thehigher-J_(c) barrier). This is because, just as in a trilayer stack,current flows “vertically” through the layers of a pentalayer stack. Inan individual Josephson junction that includes both first insulatingbarrier 2112 and second insulating barrier 2114, the two insulatingbarriers are effectively in series with one another and the criticalcurrent of the junction is determined by the lower of the two J_(c)s.Due to the nature of the photoresist masking and etching techniques usedto pattern Josephson junctions, it is straightforward to remove thetopmost insulating barrier (i.e., second insulating barrier 2114) from apentalayer stack while leaving the bottommost insulating barrier (i.e.,first insulating barrier 2112) in place to define a Josephson junction,but it is considerably more difficult to remove the bottommostinsulating barrier while leaving the topmost insulating barrier inplace. Therefore, a Josephson junction patterned in pentalayer 2110 willgenerally include either; i) both first insulating barrier 2112 andsecond insulating barrier 2114 such that the critical current of thejunction is determined by the insulating barrier with the lower J_(c)(i.e., by second insulating barrier 2114), or ii) only first insulatingbarrier 2112 such that the critical current of the junction isdetermined by insulating barrier 2112. When only first insulatingbarrier 2112 is present, the critical current of the Josephson junctionis determined by the J_(c) of first insulating barrier 2112. However,when both first insulating barrier 2112 and second insulating barrier2114 are present, the critical current of the Josephson junction isdetermined by the lower of the two J_(c)s (in this case, the criticalcurrent is determined by second insulating barrier 2114). Thus, it isadvantageous for the J_(c) of the topmost insulating barrier (i.e.,second insulating barrier 2114) to be lower than the J_(c) of thebottommost insulating barrier (i.e., first insulating barrier 2112) toenable Josephson junctions of two different critical currents to beformed. If the bottommost insulating barrier (i.e., first insulatingbarrier 2112) had the lower J_(c) of the two, then only junctions havinga critical current defined by the bottommost insulating barrier (i.e.,first insulating barrier 2112) could practically be formed.

FIG. 21B is a sectional view of a portion of an exemplarysuperconducting integrated circuit 2100 b in accordance with the presentsystems and methods. FIG. 21B depicts superconducting integrated circuit2100 a from FIG. 21A after two Josephson junctions 2121 and 2122 havebeen defined using pentalayer 2110. Junction 2121 includes patternedportions of both first insulating barrier 2112 and second insulatingbarrier 2114, whereas junction 2122 includes only a patterned portion offirst insulating barrier 2112. Thus, the critical current of junction2122 is determined by the J_(c) of first insulating barrier 2112, whilethe critical current of junction 2121 is determined by the lower J_(c)between that of first insulating barrier 2112 and that of secondinsulating barrier 2114. Second insulating barrier 2114 is thicker thanfirst insulating barrier 2112 and therefore second insulating barrier2114 has a lower J_(c) than first insulating barrier 2112. The criticalcurrent of junction 2121 is therefore determined by the J_(c) of secondinsulating barrier 2114. Note, however, that if the J_(c) of secondinsulating barrier 2114 were higher than that of first insulatingbarrier 2112, then the critical current of junction 2121 would bedetermined by the J_(c) of first insulating barrier 2112 and bothjunction 2121 and junction 2122 would have the same critical current(for the same junction area) despite the presence of second insulatingbarrier 2114.

An example of a superconducting integrated circuit in which it isdesirable to include Josephson junctions having substantially differentcritical currents is a superconducting quantum processor having local,on-chip memory and/or control circuitry. In such a circuit,superconducting qubits may employ Josephson junctions having a firstcritical current (or first range of critical currents) and on-chipmemory/control circuitry may employ Single Flux Quantum (SFQ), QuantumFlux Parametron (QFP), or other superconducting logic circuitry(including but not limited to the schemes described in U.S. Pat. Nos.8,098,179, 7,876,248, 8,035,540, 7,843,209, 8,018,244, and US PatentPublication Number 2011-0065586, each of which is incorporated herein byreference in its entirety) that may employ Josephson junctions having asecond critical current (or second range of critical currents) that issubstantially different from the first critical current (or first rangeof critical currents). By employing a Josephson junction pentalayer,memory/control circuitry may be integrated into the quantum processorarchitecture without requiring large-area Josephson junctions, and suchmay reduce the area of the processor and, for example, enable qubit sizeto be minimized (qubit size is advantageously minimized in quantumprocessors in order to reduce the coupling of noise into the qubitcircuits).

FIG. 22 shows a method 2200 for forming a Josephson junction pentalayerin accordance with the present systems and methods. Method 2200 includesfive operations or acts 2201-2205, though those of skill in the art willappreciate that in alternative embodiments certain acts may be omittedand/or additional acts may be added. Those of skill in the art willappreciate that the illustrated order of the acts is shown for exemplarypurposes only and may change in alternative embodiments. At 2201, afirst superconducting metal layer is deposited. The firstsuperconducting metal layer may comprise, for example, niobium and maybe deposited on a dielectric layer or substrate. At 2202, a firstinsulating barrier is deposited over the first superconducting metallayer. The first insulating barrier may comprise, for example, aluminumoxide and depositing the first insulating barrier over the firstsuperconducting metal layer may include depositing a layer of aluminumon top of the first superconducting metal layer and growing a layer ofaluminum oxide on the layer of aluminum. The first insulating barriermay have a first thickness that provides a first critical currentdensity, J_(c1). At 2203, a second superconducting metal layer isdeposited over the first insulating barrier. The second superconductingmetal layer may comprise, for example, niobium. At 2204, a secondinsulating barrier is deposited over the second superconducting metallayer. The second insulating barrier may comprise, for example, aluminumoxide and depositing the second insulating barrier over the secondsuperconducting metal layer may include depositing a layer of aluminumon top of the second superconducting metal layer and growing a layer ofaluminum oxide on the layer of aluminum. The second insulating barriermay have a second thickness that provides a second critical currentdensity, J_(c2). At 2205, a third superconducting metal layer isdeposited over the second insulating barrier. The third superconductingmetal layer may comprise, for example, niobium. In accordance with thepresent systems and methods, the second thickness of the secondinsulating barrier may be substantially different from the firstthickness of the first insulating barrier (such that J_(c1) issubstantially different from J_(c2)) in order to facilitate thefabrication of Josephson junctions having substantially differentcritical currents in, for example, applications of superconductingquantum processors. As described previously, it may be advantageous toensure that the second thickness of the second insulating barrier issubstantially greater than the first thickness of the first insulatingbarrier such that the critical current density J_(c2) of the secondinsulating barrier is substantially less than the critical currentdensity J_(c1) of the first insulating barrier. The pentalayer formed bymethod 2200 may then be patterned to form Josephson junctions havingsubstantially different critical currents without requiring that theJosephson junctions have substantially different areas in accordancewith the present systems and methods.

Certain aspects of the present systems and methods may be realized atroom temperature, and certain aspects may be realized at asuperconducting temperature. Thus, throughout this specification and theappended claims, the term “superconducting” when used to describe aphysical structure such as a “superconducting metal” is used to indicatea material that is capable of behaving as a superconductor at anappropriate temperature. A superconducting material may not necessarilybe acting as a superconductor at all times in all embodiments of thepresent systems and methods.

The above description of illustrated embodiments, including what isdescribed in the Abstract, is not intended to be exhaustive or to limitthe embodiments to the precise forms disclosed. Although specificembodiments of and examples are described herein for illustrativepurposes, various equivalent modifications can be made without departingfrom the spirit and scope of the disclosure, as will be recognized bythose skilled in the relevant art. The teachings provided herein of thevarious embodiments can be applied to other superconductive circuits andstructures, not necessarily the exemplary superconductive circuits andstructures generally described above.

The teachings of U.S. provisional patent application Ser. No. 61/608,379filed Aug. 3, 2012 and U.S. provisional patent application Ser. No.61/714,642 filed Oct. 16, 2012 are incorporated herein by reference, intheir entirety.

The various embodiments described above can be combined to providefurther embodiments. To the extent that they are not inconsistent withthe specific teachings and definitions herein, all of the U.S. patents,U.S. patent application publications, U.S. patent applications, foreignpatents, foreign patent applications assigned D-Wave Systems Inc.referred to in this specification and/or listed in the Application DataSheet, are incorporated herein by reference, in their entirety. Aspectsof the embodiments can be modified, if necessary, to employ systems,circuits and concepts of the various patents, applications andpublications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

The invention claimed is:
 1. A superconducting integrated circuitcomprising: a first superconducting metal layer; a first insulatingbarrier having a first thickness, wherein the first insulating barrieris positioned over the first superconducting metal layer; a secondsuperconducting metal layer positioned over the first insulatingbarrier; a second insulating barrier having a second thickness, whereinthe second insulating barrier is positioned over the secondsuperconducting metal layer; and a third superconducting metal layerpositioned over the second insulating barrier; a dielectric layerpositioned over the third superconducting metal layer; a superconductingwiring layer positioned over the dielectric layer; and at least onesuperconducting via that superconductingly electrically couples at leasta portion of the superconducting wiring layer to at least a portion ofat least one of the second superconducting metal layer and the thirdsuperconducting metal layer.
 2. The superconducting integrated circuitof claim 1 wherein the second thickness of the second insulating barrieris greater than the first thickness of the first insulating barrier. 3.The superconducting integrated circuit of claim 1 wherein at least afirst portion of the superconducting integrated circuit is patterned toform a first Josephson junction comprising: a first portion of the thirdsuperconducting metal layer; a first portion of the second insulatingbarrier; a first portion of the second superconducting metal layer; afirst portion of the first insulating barrier; and a first portion ofthe first superconducting metal layer, and wherein at least onesuperconducting via superconductingly electrically couples a firstportion of the superconducting wiring layer to the first portion of thethird superconducting metal layer.
 4. The superconducting integratedcircuit of claim 3 wherein at least a second portion of thesuperconducting integrated circuit is patterned to form a secondJosephson junction comprising: a second portion of the secondsuperconducting metal layer; a second portion of the first insulatingbarrier; and a second portion of the first superconducting metal layer,and wherein at least one superconducting via superconductinglyelectrically couples a second portion of the superconducting wiringlayer to the second portion of the second superconducting metal layer.5. The superconducting integrated circuit of claim 1 wherein at least afirst portion of the superconducting integrated circuit is patterned toform a first Josephson junction comprising: a first portion of thesecond superconducting metal layer; a first portion of the firstinsulating barrier; and a first portion of the first superconductingmetal layer, and wherein at least one superconducting viasuperconductingly electrically couples a first portion of thesuperconducting wiring layer to the first portion of the secondsuperconducting metal layer.
 6. The method of claim 1 wherein the firstinsulating barrier comprises: a first layer of silicon nitride thatdirectly overlies the first superconducting metal layer; a layer ofsilicon dioxide that directly overlies the first layer of siliconnitride; and a second layer of silicon nitride that directly overliesthe layer of silicon dioxide.
 7. The superconducting integrated circuitof claim 5 wherein at least a second portion of the superconductingintegrated circuit is patterned to form a second Josephson junctioncomprising: a first portion of the third superconducting metal layer; afirst portion of the second insulating barrier; and a second portion ofthe second superconducting metal layer, and wherein at least onesuperconducting via superconductingly electrically couples a secondportion of the superconducting wiring layer to the first portion of thethird superconducting metal layer.
 8. The superconducting integratedcircuit of claim 7 wherein the first Josephson junction has a firstcritical current density, the second Josephson junction has a secondcritical current density, and the first thickness of the firstinsulating layer and the second thickness of the second insulated layerare selected such that the first critical current density of the firstJosephson junction is less than the second critical current density ofthe second Josephson junction.
 9. The superconducting integrated circuitof claim 4 wherein the first Josephson junction has a first criticalcurrent density, the second Josephson junction has a second criticalcurrent density, and the first thickness of the first insulating layerand the second thickness of the second insulated layer are selected suchthat the first critical current density of the first Josephson junctionis less than the second critical current density of the second Josephsonjunction.